addons: bridge: identify a bridge with bridge-ports or bridge-vlan-aware keywords

addons: bridge: identify a bridge with bridge-ports or bridge-vlan-aware keywords

Ticket: CM-14158
Reviewed By: julien, nikhilg
Testing Done: Tested with a bridge config without bridge-ports line.

NCLU wants the ability to create a bridge without ports (to add them
later). Today we cannot specify an attribute without values. We get the
below error:
info: processing interfaces file /etc/network/interfaces
error: /etc/network/interfaces: line10: iface bridge: invalid syntax
'bridge-ports'

The error comes from a generic parser check...we will
need to add exceptions to the generic check if we allow attributes
without values.

This patch simply identifies a bridge by both the bridge-vlan-aware and
bridge-ports keyword. For vlan aware bridge it will simply work.
For old bridge driver nclu will have to specify 'bridge-vlan-aware no'
to achieve the same. nclu does not support old bridge
model today, so defering the old bridge driver discussion for later.

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