Adds help messages to mongodb_store_db and mongodb_store_uri
Adds help messages to mongodb_store_db and mongodb_store_uri

Help messages are based on the webpages below:

http://api.mongodb.org/python/current/api/pymongo/mongo_client.html#pymongo.mongo_client.MongoClient
http://api.mongodb.org/python/current/api/gridfs/

DocImpact

Change-Id: Id95459b3eb2b50703e5793e3bf02c9ea1b142236

redhat-openstack/glance,akash1808/glance,ozamiatin/glance,saeki-masaki/glance,SUSE-Cloud/glance,rickerc/glance_audit,openstack/glance,dims/glance,citrix-openstack-build/glance,stevelle/glance,rickerc/glance_audit,redhat-openstack/glance,rajalokan/glance,ozamiatin/glance,darren-wang/gl,cloudbau/glance,ntt-sic/glance,takeshineshiro/glance,ntt-sic/glance,paramite/glance,sigmavirus24/glance,kfwang/Glance-OVA-OVF,citrix-openstack-build/glance,SUSE-Cloud/glance,cloudbau/glance,jumpstarter-io/glance,scripnichenko/glance,klmitch/glance,jumpstarter-io/glance,tanglei528/glance,takeshineshiro/glance,dims/glance,scripnichenko/glance,wkoathp/glance,paramite/glance,openstack/glance,vuntz/glance,vuntz/glance,tanglei528/glance,sigmavirus24/glance,akash1808/glance,darren-wang/gl,rajalokan/glance,stevelle/glance,kfwang/Glance-OVA-OVF,klmitch/glance,wkoathp/glance,JioCloud/glance,openstack/glance,JioCloud/glance,saeki-masaki/glance