Connecting REST API to DB, part I

    * Implemented Clusters POST URI functionality which creates a
      new cluster
    * Added new create_cluster function in DB API
    * Added new get_clusters_nodes function in DB API
    * Updated DB scema
    ** Removed endpoint_types table and merged content in endpoints
       table
    ** Added volume_size column in clusters and nodes tables
    ** Added deleted (boolean) column in clusteers, nodes and endpoints
       tables
    * Added 'add' class method in DB Cluster, Node and Endpoint classes

Change-Id: Ie7f03dc0a0d9d143a212b56be179d4f2a162c75b