Remove unused 'as e' in exception blocks

Change-Id: I0bd26e1657a4e0ec40405d03d43aee9e10a13636
Closes-bug: #1287031
Embrane Plugin fails alembic migrations

Alembic migration from revision f44ab9871cd6 to 2eeaf963a447 is failing on
Embrane Plugin because the floatingips table doesn't exist.

The problem happens because the plugin is actually inheriting from OVS's Plugin,
and therefore it doesn't take part of the correct migration path at installation time.

As far as I have investigated, adding the support on ext_gw_mode and l3_support is enough to solve the problem.

Change-Id: Ide740df34d06d87fab50cf6468c5d753cb46f112
Closes-bug: 1287330
Merge "Embrane Plugin fails alembic migrations"
Merge "Remove unused 'as e' in exception blocks"