Bug 1125423 - Fuse Fabric Groups Plugin cannot be enabled

Generally when there is an attempt to enable server plugins we now first
check it's configuration by looking for required properties (in ConfigDef)
and making sure there's valid value in configuration. If something is
missing, it means plugin is not configured and cannot be enabled.
ServerPluginManagerBean thows this using
PluginConfigurationRequiredException that is then presented to user.

(cherry picked from commit bd1095fa5ac2f55544d5f8c4b45033b4050b94d9)
Signed-off-by: Thomas Segismont <tsegismo@redhat.com>
Bug 1125423 - Fuse Fabric Groups Plugin cannot be enabled

Fixed failing itest and also added test

(cherry picked from commit c4fc397e59325e17fca9a06af93acd2d79015f20)
Signed-off-by: Thomas Segismont <tsegismo@redhat.com>