Configuration file plugins support for nailgun

    - Task serialization logic for pre/post hooks
    - PluginSerializer object added
    - API for interaction with Plugins db schema
    General API requests supported
    GET/POST /plugins
    GET/PUT/DELETE /plugins/<plugin_id>
    - Plugins and ClustersPlugins db models added
    ClusterPlugins used to identify enabled plugins for a
    given cluster
    - Configuration uploading logic stored in Plugin wrapper

Co-authored-by: Dima Shulyak <dshulyak@mirantis.com>
Co-authored-by: Evgeniy L <eli@mirantis.com>

implements: blueprint cinder-neutron-plugins-in-fuel
Change-Id: I986f5be9ed3c3adaf7583d1bfc546cbe705db9ec
Merge "Configuration file plugins support for nailgun"