Rework VRF view to allow viewing default VRF

Rewrite '-' or 'none' to mean "the default VRF" when trying to list
VRFs. Also add check that user has actually entered a RT before trying
to search.

Fixes #527.
Merge pull request #541 from plajjan/527-view-vrf-default

Rework VRF view to allow viewing default VRF