addons: vrf: don't delete vrf map file at init if mgmt vrf already exists

addons: vrf: don't delete vrf map file at init if mgmt vrf already exists

Ticket: CM-12084
Reviewed By: julien, nikhil
Testing Done: tested with --allow=mgmt class

This fixes a problem where vrf map file was getting deleted
at boot during the second invocation of ifup. This is because
the code uses PERFMODE to check it is boot..but does
not realize that this may not be the first invocation of
ifup during the boot.

This patch adds additional checks for mgmt netdev to
make sure we delete the vrf map file only the first time.

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