Merge duplicate code for network quotas tests

There is some duplicate code between test_quotas and test_lbaas_quotas.
To clearify the difference between them and the purpose, this patch merges
most code of them into a single internal method.

Change-Id: I05a9c92d052ea2f99694a588c2a3cebbde625166
Skip migrate server invalid state if resize is off

The migrate (aka cold migration) can be considered a special case of the
resize with a flavor_id equals to None. For that reason to perform
the migrate tests we need to be sure that the resize is enabled.
This patch adds a skip decorators to verify that.

Change-Id: I3a02b1bd210e3934dfd61d4182b301cc33d8a962
Merge "Skip migrate server invalid state if resize is off"
Merge "Merge duplicate code for network quotas tests"