Nexus: Common neutron constants moved to plugin constants

Per bug 1441043, the common neutron constants like MAX_VLAN_TAG has been moved to plugins.common.constants.
This is failing Jenkins on networking-cisco for the nexus driver as it depends on those constants.

Traceback (most recent call last):
2015-04-14 18:50:22.102 | File "/home/jenkins/workspace/gate-networking-cisco-python27/.tox/py27/local/lib/python2.7/site-packages/unittest2/loader.py", line 445, in _find_test_path
2015-04-14 18:50:22.103 | module = self._get_module_from_name(name)
2015-04-14 18:50:22.103 | File "/home/jenkins/workspace/gate-networking-cisco-python27/.tox/py27/local/lib/python2.7/site-packages/unittest2/loader.py", line 384, in _get_module_from_name
2015-04-14 18:50:22.103 | __import__(name)
2015-04-14 18:50:22.103 | File "networking_cisco/tests/unit/ml2/drivers/cisco/nexus/test_type_nexus_vxlan.py", line 18, in <module>
2015-04-14 18:50:22.103 | from networking_cisco.plugins.ml2.drivers.cisco.nexus import type_nexus_vxlan
2015-04-14 18:50:22.103 | File "networking_cisco/plugins/ml2/drivers/cisco/nexus/type_nexus_vxlan.py", line 39, in <module>
2015-04-14 18:50:22.103 | MIN_NEXUS_VNI = n_const.MAX_VLAN_TAG + 2
2015-04-14 18:50:22.103 | AttributeError: 'module' object has no attribute 'MAX_VLAN_TAG'
2015-04-14 18:50:22.103 | The test run didn't actually run any tests