fix: ensure the hosts definition is always defined

Ensure that the `hosts` definition is always defined, defaulting to an
empty set. Due to the lack of an Ansible inventory file this is most
likely meant to be set as an Ansible extra variable [1].

We also rename the variable named `hosts` to `my_hosts` because `hosts`
is a reserved name in Ansible. Reserved names in Ansible are for example
all class variable names of plays, roles, blocks, and tasks [2], and
`hosts` for example is used by plays [3].

[1] https://docs.ansible.com/ansible/latest/playbook_guide/playbooks_variables.html#defining-variables-at-runtime
[2] https://github.com/ansible/ansible/blob/v2.16.4/lib/ansible/vars/reserved.py#L39
[3] https://github.com/ansible/ansible/blob/v2.16.4/lib/ansible/playbook/play.py#L58

ansible/configuration/fail2ban/config-f2b-protect-sshd.yaml

@@ -1,6 +1,6 @@
 ---
 - name: install fail2ban and configure sshd
-  hosts: "{{ hosts }}"
+  hosts: "{{ my_hosts | d([]) }}"
   become: yes
   tasks:
   - name: install fail2ban

ansible/configuration/ssh/config-add-sshkey.yaml

@@ -1,6 +1,6 @@
 ---
 - name: add ssh key
-  hosts: "{{ hosts }}"
+  hosts: "{{ my_hosts | d([]) }}"
   become: yes
   tasks:
   - name: install public keys

ansible/deployment/portainer/deploy-portainer.yaml

@@ -1,6 +1,6 @@
 ---
 - name: deploy portainer-ce latest
-  hosts: "{{ hosts }}"
+  hosts: "{{ my_hosts | d([]) }}"
   become: yes
   become_user: "{{ lookup('env','USER') }}"
   tasks:

ansible/deployment/traefik/deploy-traefik.yaml

@@ -1,6 +1,6 @@
 ---
 - name: deploy traefik v2.5
-  hosts: "{{ hosts }}"
+  hosts: "{{ my_hosts | d([]) }}"
   become: yes
   tasks:
   - name: deploy traefik

ansible/installation/inst-core.yaml

@@ -1,5 +1,5 @@
 - name: install core packages
-  hosts: "{{ hosts }}"
+  hosts: "{{ my_hosts | d([]) }}"
   become: yes
   tasks:
   - name: install core packages

ansible/installation/inst-docker-ubuntu.yaml

@@ -1,6 +1,6 @@
 ---
 - name: install docker
-  hosts: "{{ hosts }}"
+  hosts: "{{ my_hosts | d([]) }}"
   become: yes
   tasks:
     - name: install docker dependencies

ansible/installation/inst-microk8s.yaml

@@ -1,6 +1,6 @@
 ---
 - name: install microk8s
-  hosts: "{{ hosts }}"
+  hosts: "{{ my_hosts | d([]) }}"
   become: yes
   tasks:
     - name: install microk8s

ansible/installation/inst-vm-core.yaml

@@ -1,6 +1,6 @@
 ---
 - name: install core packages for virtual machines
-  hosts: "{{ hosts }}"
+  hosts: "{{ my_hosts | d([]) }}"
   become: yes
   tasks:
   - name: install packages

ansible/installation/inst-wireguard.yaml

@@ -1,6 +1,6 @@
 ---
 - name: install wireguard
-  hosts: "{{ hosts }}"
+  hosts: "{{ my_hosts | d([]) }}"
   become: yes
   tasks:
     - name: install wireguard

ansible/installation/inst-zsh.yaml

@@ -1,6 +1,6 @@
 ---
 - name: install zsh
-  hosts: "{{ hosts }}"
+  hosts: "{{ my_hosts | d([]) }}"
   become: yes
   tasks:
     - name: install zsh

ansible/maintenance/maint-diskspace.yaml

@@ -1,6 +1,6 @@
 ---
 - name: check disk space
-  hosts: "{{ hosts }}"
+  hosts: "{{ my_hosts | d([]) }}"
   tasks:
     - name: get disk usage
       ansible.builtin.command: df -h

ansible/maintenance/maint-docker-clean.yaml

@@ -1,6 +1,6 @@
 ---
 - name: clean docker
-  hosts: "{{ hosts }}"
+  hosts: "{{ my_hosts | d([]) }}"
   tasks:
     - name: prune non-dangling images
       community.docker.docker_prune:

ansible/maintenance/maint-reboot-required.yaml

@@ -1,6 +1,6 @@
 ---
 - name: check if system reboot is required
-  hosts: "{{ hosts }}"
+  hosts: "{{ my_hosts | d([]) }}"
   become: yes
   tasks:
     - name: check if system reboot is required

ansible/maintenance/maint-reboot.yaml

@@ -1,6 +1,6 @@
 ---
 - name: reboot machine
-  hosts: "{{ hosts }}"
+  hosts: "{{ my_hosts | d([]) }}"
   become: yes
   tasks:
   - name: reboot machine

ansible/notification/notify-discord.yaml

@@ -11,7 +11,7 @@
 
 - name: notify discord
 
-  hosts: "{{ hosts }}"
+  hosts: "{{ my_hosts | d([]) }}"
 
   vars:
     # The name that will be shown as sender of the notification. Note