Add string filters to template lookups in cloud-init base template

Added | string filters to lookup() calls in the cloud-init base template
to ensure consistent string handling across all cloud providers.

The Vultr startup_script module requires all values to be JSON-serializable
strings, and lookup() can return bytes in some contexts. This change ensures
that both the SSH config template lookup and the SSH public key file lookup
explicitly return strings.

This is a defensive fix that improves compatibility with strict JSON
serialization requirements in some Ansible modules, while remaining
backward compatible with existing cloud providers.

Related to: vultr.cloud.startup_script JSON serialization requirements

Author: fank

files/cloud-init/base.yml

@@ -30,12 +30,12 @@ users:
     shell: /bin/bash
     lock_passwd: true
     ssh_authorized_keys:
-      - "{{ lookup('file', SSH_keys.public) }}"
+      - "{{ lookup('file', SSH_keys.public) | string }}"
 
 write_files:
   - path: /etc/ssh/sshd_config
     content: |
-{{ lookup('template', 'files/cloud-init/sshd_config') | indent(width=6, first=True) }}
+{{ lookup('template', 'files/cloud-init/sshd_config') | string | indent(width=6, first=True) }}
 
 runcmd:
   - set -x