fixed ip quota off by a large margin

[Matthew Treinish]

Occasionally an overquota exception is raised by allocate_fixed_ip when there is quite a large amount of free fixed_ip left.

For example in this tempest postgres job run:

http://logs.openstack.org/87/214987/9/check/gate-tempest-dsvm-postgres-full/88910d0/logs/screen-n-net.txt.gz#_2015-09-09_06_03_26_402

The overquota exception is raised by there are 96 free fixed ips available according to the log message. (which is pulled from the contents of the exception.

According to logstash this is fairly uncommon with only 20 hits in the past 10 days:

http://logstash.openstack.org/#eyJzZWFyY2giOiJtZXNzYWdlOlwiRml4ZWRJcExpbWl0RXhjZWVkZWRcIiBBTkQgdGFnczpcInNjcmVlbi1uLW5ldC50eHRcIiIsImZpZWxkcyI6W10sIm9mZnNldCI6MCwidGltZWZyYW1lIjoiYWxsIiwiZ3JhcGhtb2RlIjoiY291bnQiLCJ0aW1lIjp7InVzZXJfaW50ZXJ2YWwiOjB9LCJzdGFtcCI6MTQ0MTg0MDkzNDM1OX0=