Disables the clear text password UI by default

The old way of assigning passwords in clear text to instances has been
superseded by the nova get-password feature. A corresponding UI
feature has been recently included in Horizon (see blueprint
"decrypt-and-display-vm-generated-password").

This commit disables the visualization of the old password UI by default.

Change-Id: Ia9a8b5c1c398054fb3d726b93f7fecc332b02efe
Closes-bug: #1291006
Merge "Disables the clear text password UI by default"