Update xmlrpc.py to add vrf argument to find_free_prefix

Fixes find_free_prefix XMLRPC, VRF was not being passed through
Merge pull request #374 from tmenari/patch-1

Update xmlrpc.py to add vrf argument to find_free_prefix
Implement growing and shrinking pools

.. using 2 methods

Using 2 methods we know beforehand if the user wants to remove a prefix
from a pool, and incase he/she wants completion, *he can opt for not specifying a range.
Commit plajjan's gist

https://gist.github.com/plajjan/91c310603c4339e2879e