Fix nova-manage commands that do not exist
Fix nova-manage commands that do not exist

The following commands do not exist.
But they appear in help message.
So fix them.

* nova-manage db print_dict
* nova-manage cell parse_server_string

Change-Id: I7b16b8ab36b9a9ae719bf98a75511d82041d0152
Closes-Bug: #1723982

mahak/nova,phenoxim/nova,gooddata/openstack-nova,gooddata/openstack-nova,openstack/nova,rahulunair/nova,mikalstill/nova,rahulunair/nova,klmitch/nova,gooddata/openstack-nova,rahulunair/nova,openstack/nova,klmitch/nova,klmitch/nova,phenoxim/nova,mikalstill/nova,mikalstill/nova,gooddata/openstack-nova,mahak/nova,mahak/nova,klmitch/nova,openstack/nova