addons: vrf: fix a few vrf enslavement and table id allocation corner cases

addons: vrf: fix a few vrf enslavement and table id allocation corner cases

Ticket: CM-9957
Reviewed By: dsa, julien, nikhil
Testing Done: Tested vrf enslave/deslave + ifreload

This patch fixes a few corner cases:
- release dhcp on all new enslavement or change of enslavement
- fix a NoneType error on ifreload when a vrf enslavement was removed
- handle a corner case with auto table ids

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>