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
Merge "Merge duplicate code for network quotas tests"
Merge branch 'master' of https://github.com/openstack/tempest