[ML2] l2-pop MD handle multi create/delete ports

[ML2] l2-pop MD handle multi create/delete ports

If more than one port is added or removed simultaneously, port db entry
have status BUILD or DOWN and pass to ACTIVE when agent have finish to
configured it.
l2-pop mechanism driver use events port pass to ACTIVE or DOWN to send
fdb entries. In case of port is the first or the last network port on
an agent, the flooding entry need to be add or removed.

This patch fix the method to determine how many ports are active on a
agent by adding filter on status port to be ACTIVE.

Closes-bug: #1263881
Change-Id: I9c1f8bd69dee37bc01a5d42327aa5f737998c5aa