Add 'server_default' parameter

Add 'server_default' parameter

If a model mentions 'default' it is better to set 'server_default'
as well because this 'defaut' can be reached only via code. This
change adds 'server_default' parameter to models and in database,
so if someone will use SQL query it will also use default value.

For model Router added parameter server_default. In migration it was
added by this change https://review.openstack.org/105212.

For model MeteringLabelRule added parameter server_default.
In migration it will be added by heal script due to problems
with Metering plugin and grenade.

Partial-Bug: #1295539

Change-Id: Iab6fb4eca73e4918c16dac2945fd8552ae2a0c73