Disable the NumericLiterals cop for rubocop

- Disable the NumericLiterals cop in the .rubocop.yml file - this cop enforces
  an underscore '_' every 3 characters of an integer. This doesn't make much
  sense in our use case and reduces readability. Especially for port numbers.
- Remove any existing entries that were abiding by the NumericLiterals
  cop.

Change-Id: I4dd9485c0defcf896945f805ca8897011c0424ab
Addresses: blueprint rubocop-integer
only add service_plugins option if there are some plugins defined

Fixes: bug 1284829

Only add a service_plugins option to the neutron.conf file if you have
defined at least one plugin in the attribute
['openstack']['network']['service_plugins']

Change-Id: Ia13593ad2bfe6ad7904650e3835b074c4281f8be