sql: add AVP column for VRFs
backend: add AVP support for VRFs
pynipap: add AVP support for VRFs
cli: fix AVP support for prefix on from-pool

Previous work and PR for adding AVPs to prefixes missed out on adding it
to the from_pool operation, since that is split from the rest of the
add_prefix functionality. We really should have one parsing function for
it though to avoid these kinds of issues in the future.
cli: add AVP support for VRFs

This is a complete implementation that introduces new commands;
 - nipap vrf modify X add extra-attribute foo=bar
 - nipap vrf modify X set extra-attribute foo=bar
 - nipap vrf modify X remove extra-attribute foo
... to add, modify or remove AVPs (or 'extra-attribute' as they are
called in the CLI).
cli: Fix AVP modification for prefixes

It's now possible to add and set extra-attributes using;
 - nipap address modify X set extra-attribute foo=bar
sql: add AVPs for pools
backend: add AVP support for pools
pynipap: add AVP support for pools
cli: add AVP support for pools
Update test cases for VRF and pool AVPs