Default to place prefix in same VRF

When the user allocates a prefix from another prefix, the new prefix's
VRF will default to the VRF of the prefix it's allocated from. To
simplify things the JSON encoded prefixes returned from the XHR
functions now include both VRF ID and RT.

Part of #286
Prefix's VRF now defaults to pool's implied VRF

When allocating a new prefix from a pool, the prefix will by default be
placed in the pool's implied VRF.

Fixes #286