Switch test_mtu.py to tempest.common.utils.requires_ext
Switch test_mtu.py to tempest.common.utils.requires_ext

The old (tempest.test.requires_ext) location is deprecated in Pike.
Somehow missed in a previous change.

Change-Id: I8e18732443b1f01649a0a98a4765dafc5dc9bb7a

openstack/neutron,mahak/neutron,mahak/neutron,eayunstack/neutron,huntxu/neutron,eayunstack/neutron,openstack/neutron,openstack/neutron,mahak/neutron,noironetworks/neutron,noironetworks/neutron,huntxu/neutron