Delete duplicated code from router tests

Reorganize duplicate code in a mixin for factorization purpose.

Change-Id: Ie5212a487a4e82c3b5e8808e04044ec38383e115
Make 'IP address' field optional for LBaaS VIP creation

Since 'IP address' argument is optional in the CLI command, it
should not be mandatory for the field "Specify a free IP address
from the selected subnet". This patch sets 'required=False' for
this field. Some LBaaS VIP test data cleanup is also done.

Closes-Bug: #1386616

Change-Id: I112919d12ca3b4f40d9b907b7f47b64502d5ae7c
Imported Translations from Transifex

For more information about this automatic import see:
https://wiki.openstack.org/wiki/Translations/Infrastructure

Change-Id: I99bb09c2b5c4c46bb7ededbbf54117f30e7c50b8
Merge "Make 'IP address' field optional for LBaaS VIP creation"
Merge "Delete duplicated code from router tests"