cephkeyring POSIX group: use usermod

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>

spec/classes/cloud_volume_storage_spec.rb

@@ -107,6 +107,7 @@ describe 'cloud::volume::storage' do
           :os_auth_url    => 'http://keystone.host:5000/v2.0'
         )
         should contain_group('cephkeyring').with(:ensure => 'present')
+        should contain_exec('add-cinder-to-group').with(:command => 'usermod -a -G cephkeyring cinder')
       end
     end