Adds backup/restore support for mongodb

Adds support to do full backups and restore mongodb single instance

Changes:

Added mongo_impl for backup and restore strategies
Updated backupagent.py to load backup_namespace and
        restore_namespace from config group section
Updated manager to invoke backup and restore
Updated common/cfg.py to load group attribs (backup_namespace,
                restore_namespace etc under [mongodb], [mysql]
Increased timeout for utils.execute() that execs mongorestore cmd

Co-Authored-By: Ramashri Umale <rumale@ebaysf.com>
Implements: blueprint backups-single-instance-mongodb
Change-Id: I67e0bc0890f64744a39a3ce097a4887f2c6a4174