Fix service subnet capacity within service_instance module

Module 'service_instance' creates service_subnet per neutron_subnet,
current mask is 29, that allows take only 3 (three) ips for service
instances. Changed mask to 28, that allows take 11 (eleven) ips now
by default.
It allows us to use multibackend installation with drivers, that
depend on this module more reliably and provides possibility to
change this value in general.

Changes:
- added config option for network subnets mask with default value 28,
  instead of hardcoded 29.
- fixed related unittests

Change-Id: Ib58048832287a9f184317d2c92f0cc22e7f2a303
Closes-Bug: #1341711
Related-Bug: #1330391
Merge "Fix service subnet capacity within service_instance module"