Add checks to verify that the plugin has been loaded

Provides a nice error message to the user: otherwise the only
sign that something went wrong is a series of warnings from
Ansible about the inventory, and then presumably some undefined
variable errors later on...

playbooks/init-credentials.yml

@@ -14,6 +14,9 @@
   gather_facts: no
   become: no
   tasks:
+    # Check that "float" is properly set up.
+    - include_role: float-plugin-check
+
     # Create the paths below credentials_dir that we're going to use.
     - name: "Create paths below {{ credentials_dir }}"
       file:

plugins/inventory/float.py

@@ -530,6 +530,7 @@ def run_scheduler(config):
     all_vars = inventory.setdefault('group_vars', {}).setdefault('all', {})
     all_vars.update(config['vars'])
     all_vars.update({
+        'float_plugin_loaded': True,
         'services': services,
         'default_service_credentials': DEFAULT_SERVICE_CREDENTIALS,
         'float_global_dns_map': _global_dns_map(inventory),

roles/base/meta/main.yml

+---
+dependencies:
+  - { role: float-plugin-check }

roles/float-plugin-check/tasks/main.yml

+---
+
+- fail:
+    msg: "The 'float' plugin was not loaded! This could be due to multiple reasons: wrong paths in ansible.cfg, problems with the Python version used by Ansible, etc..."
+  when: "not float_plugin_loaded | default(False)"
+