add a workaround for running with --noop or --tags

This fixes errors like:
Error: Could not prefetch rabbitmq_user provider 'rabbitmqctl': Command rabbitmqctl is missing

...under the following conditions:
* The client does not yet have rabbitmq installed.
* Puppet is run with --noop or a specific set of --tags that does not
  include rabbitmq resources.

On a fresh client in normal operation, the sequence of events is:
1. Install the rabbitmq package.
2. Start the service.
3. Run the prefetch for resources which use rabbitmqctl (e.g. rabbitmq_user).
4. Apply the resources (or not, if they already exist).

On a fresh client running with --noop or tags, the sequence of events is:
1. (nothing)
2. (nothing)
3. Run the prefetch, but fail.

To work around this, assume that if rabbitmq management commands are not
found, then rabbitmq is not installed at all, thus all corresponding
resources are not present.

On a normal run, the rabbitmq packages will be installed before
resources which need rabbitmq commands, so the workaround is never
triggered.

This is _probably_ not an ideal fix, but it's the best I have.

Author: bugfood

lib/puppet/provider/rabbitmq_cli.rb

@@ -54,6 +54,14 @@ def self.rabbitmqctl_list(resource, *opts)
         ['-q']
       end
     rabbitmqctl("list_#{resource}", *list_opts, *opts)
+  rescue Puppet::MissingCommand
+    # rabbitmqctl is not present. Normally we would have installed a package
+    # that provides rabbitmqctl by now, but if we're running under --noop or
+    # with a restrictive set of tags, the package may not have been installed.
+    # Return an empty string to avoid error. This may give false positives for
+    # resources under --noop!
+    Puppet.debug('rabbitmqctl command not found; assuming rabbitmq is not installed')
+    ''
   end
 
   def self.rabbitmq_running

lib/puppet/provider/rabbitmq_plugin/rabbitmqplugins.rb

@@ -18,6 +18,11 @@ def self.plugin_list
       # To preserve idempotency we should get all enabled plugins regardless of implicitly or
       # explicitly enabled.
       rabbitmqplugins('list', '-e', '-m')
+    rescue Puppet::MissingCommand
+      # See note about Puppet::MissingCommand in:
+      # lib/puppet/provider/rabbitmq_cli.rb
+      Puppet.debug('rabbitmqplugins command not found; assuming rabbitmq is not installed')
+      ''
     end
     # Split by newline.
     lines = list_str.split(%r{\n})