addons: bridge: Fix KeyError when 'bridge-pvid' is not in the set 'bridge-vid'

addons: bridge: Fix KeyError when 'bridge-pvid' is not in the set 'bridge-vid'

Ticket: CM-11998
Reviewed By: roopa
Testing Done: used same config given in bug descreption

When we try to unconditionally remove 'pvid' from 'vids' set,
we run into KeyError if 'vids' set doesn't contain 'pvid'

Example configuration:
auto swp9
iface swp9
   mstpctl-portautoedge no

auto frontnet
iface frontnet
   bridge-vlan-aware yes
   bridge-ports swp9
   bridge-vids 10
   bridge-stp on

Signed-off-by: Nikhil <nikhil@cumulusnetworks.com>