Get the vCenter password from databag for compute

1. Add new attribute openstack_vmware_secret_name to indicate the
databag secret key
2. Get the password from databag if dev mode is false
3. Update the test cases

Implements: blueprint vmware-password-databag

Change-Id: I342e26aee713216f2aecc39113a67ada25ec1fb5
Merge "Get the vCenter password from databag for compute"