Add cleaup_db command to ostf-server

For ostf-server 'cleanup_db' command was added thus file cli_config and
adapter_api were modified in order to provide new behaivour of
ostf-server when mentioned command is supplied. New function clean_up_db
was added to nailgun_hooks.py file. This function performs cleaning
action for db.

Change-Id: Ifc88c2d3b617cc9e8f575182e5d4917280e9b7a2