Make fields for password injection configurable

Nova can inject root passwords into instances. Not all hypervisors support
that feature and visibility of the password fileds can get configured.

The relevant patch is not upstreamable, so this is only effective when
the dashboard package from the openSUSE build service is used.

Configure nova_dashboard to not disply the password fields.

Bugzilla entry: https://bugzilla.novell.com/show_bug.cgi?id=869696
Merge pull request #169 from nkrinner/password-field-3

Make fields for password injection configurable