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
(cherry picked from commit - db9c08e1721710d3b8f866a47ec1034bf2cf7d9d)

Backported-from: icehouse
Upstream-Review: https://review.openstack.org/#/c/56151/
Closes-rally-bug: DE230
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superseded by the nova get-password feature.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
(cherry picked from commit 5d6fe4b05f5ee768d353f02ccbe396491ac27135)

Backported-from: icehouse
Upstream-Review: https://review.openstack.org/#/c/79736/
Closes-rally-bug: DE230