Add a pool_group to pools in v1.1

Pools can't be grouped together based on type, region or anything. This
is bad for a couple of things but probably the most critical effect is
that when adding pools of different types, the algorithm will balance
across all existing pools regardless of their type.

This patch adds a group field to pools. The group field's default value
is None so that it is backwards compatible for v1 and to avoid forcing
operators to set a group when they simply don't care.

In addition to the above, this patch makes flavors use the pool group
instead of the pool name. This allow us to have multiple pools baking a
specific flavor. Moreover, this also allows us to balance a flavor
across of registered pools and it's the base of the future queue
migrations work.

Change-Id: Ibeba39725ddbc1bc82d4e9d5bceff8b9d69d5839
Closes-bug: #1373994
Merge "Add a pool_group to pools in v1.1"