Fix exception raised when a requested console type is disabled

The get_{vnc/rdp/spice}_console calls should raise
ConsoleTypeUnavailable not ConsoleTypeInvalid when the
corresponding console is disabled (rather than supplying an
invalid subtype).

Also removes exception handling of ConsoleTypeInvalid from the
os-remote-consoles V3 API plugin as the schema validation ensures
the input passed to nova internals is valid.

Change-Id: Ia444775ff6ec1d869d810bc689914efb8cd233a4
Merge "Fix exception raised when a requested console type is disabled"