Correctly inherit __table_args__ from parent class

Where unique constraints are added to certain tables by populating
__table_args__, we need to ensure we inherit the top level
__table_args__ that define the default storage engine to use when
creating tables.

Change-Id: Ib886b84922830b60282b4f882591e43f5041e713
Fixes: bug 1288358
OFA agent: use hexadecimal IP address in tunnel port name

The remote IP address is used to form the tunnel port name
With some OVS/Linux combinations there is a 15 character limit
on port names, so a name like 'gre-192.168.10.10' does not work.
This fix uses the shorter hexadecimal representation of
the IP address instead.
This problem was reported and fixed for OVS agent.

Closes-Bug: 1294474

Change-Id: Ieab1eb8a5c430f69aff61a317cc28f23ba95ad2d
Hide ipv6 subnet API attributes

The attributes for ra mode and address mode should be hidden
until the IPv6 feature is fully implemented.

The changes in this patch will be reverted by another patch
which closes this bug.

Change-Id: I69a1a571b5beb566641200e60b84f0716c1ec138
Related-Bug: 1304093
Merge "Correctly inherit __table_args__ from parent class"
Merge "OFA agent: use hexadecimal IP address in tunnel port name"
Merge "Hide ipv6 subnet API attributes"