rgw/dbstore: Add options to configure dbstore db file path & name_prefix
DBStore will use the below options when creating .db file for rgw server
to use.
'dbstore_db_dir':
- path for the directory to store db backend store files
- default: /var/run/ceph
'dbstore_db_name_prefix':
- prefix to be used for the files created by db backend store
- defauult: dbstore
For eg., by default the full path of the file shall be
eg., /var/run/ceph/dbstore-default_ns.db