Improve nipap-cli handling of default_vrf

The configuration option 'default_vrf' has now been replaced with two
new ones;
 - 'default_vrf_rt' only affects address adds/edits/removals and in
   which VRF to search when adding a member prefix to a pool for the
   first time - if the pool already has member prefixes, the implied VRF
   of the pool will be used.
 - 'default_list_vrf_rt' control the default VRF for listing/search
   operations, ie 'nipap address list'.

The output of VRF related information has been changed for all commands
to include both the VRF RT as well as the name. All search strings and
similar are now enclosed in single quotes when used in various messages
to make it clear what is a pool name and what is just part of the
message.

Some logic around handling the global VRF has been removed since there
is always a VRF (change made quite long ago actually).

Part of #451.
Fixes #386.
Merge branch '451-default_vrf'