Add option to disable server password fields

Not many hypervisors support setting a server admin password via Nova.
This change adds an item to OPENSTACK_HYPERVISOR_FEATURES called
'can_set_password' that controls whether password settings fields
are displayed in the UI.

Change-Id: I03399c171b2b7cbf756292cee283419d6235461f
Closes-bug: #1250713
Merge "Add option to disable server password fields"