Move values for network_type to plugins.common.constants.py

Move values for network_type to plugins.common.constants.py

It is quite confusing to have values for network type in common.constants.py
instead of having in plugins.common.constants.py.

Currently, the plugins/common/constants.py consists network_type constants
like VLAN, VXLAN, GRE etc. but values for network type like ranges
are defined in common.constants.py which is not good, it is better to have
both things at the same place.

This patch set addresses the same.

Moved out few methods which are predominantly used in plugins
from common.utils.py to plugins.common.utils.py.

Removed constants which were used in neutron-fwaas from
plugins.common.constants.py: https://review.openstack.org/#/c/168709/

Closes-Bug: #1441043

Change-Id: Iecfb15c541ed5d3cce95ba48f072af7fa60ac6f1