addons: vrf: more fixes and cleanups for mgmt and data vrf handling

addons: vrf: more fixes and cleanups for mgmt and data vrf handling

Ticket: CM-10188, CM-9881
Reviewed By: nikhil, julien, dsa, daniel
Testing Done: Tested mgmt vrf bringup, teardown, static routes at boot
etc

This patch fixes a few things:
- kill existing ssh clients on enslavement change for mgmt vrf (original patch by NikhilG)
- bring vrf master up first during vrf slave enslavement if
master does not exist. This was originally done only for
vrf dhcp slaves. With this patch we do it for all vrf slaves.
needed for static routes on vrf slaves (CM-10188).
- cleanup: reorganize code and a few cleanups and corner case handling

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