l3: not use L2 plugin _get_subnet unnecessarily

l3: not use L2 plugin _get_subnet unnecessarily

This patch is clean up to prevent future breakage by eliminating
potentially dangerous code.

l3_db and related code use L2 plugin _get_subnet and related method
unnecessarily instead of get_subnet.
It's dangerous because _get_subnet returns ORM db object which allows
the caller to update db rows directly. So the caller of _get_subnet
may update subnet db without notifying L2 plugin unintentionally.
In that case, L2 plugin or ML2 mechanism driver will be confused.
This patch replaces _get_subnet and _get_subnets_by_network with
get_subnet, get_subnets_by_network where possible.

Change-Id: I85769e639a408a292b5bd70a9d9a1ac292e2b51c
Related-Bug: #1475093