Allow to provide default password when invoke get_secret for develop mode

We need provide an option to specify a password when invoke the
get_secret instead of to return the value of index directly.
We can use the attribute node['openstack']['secret'][index] to save the
password.

Implements: blueprint vmware-password-databag
Change-Id: Ie9421b60f8a6b38a976941c1fe9c33c2962f091e