Remove unnecessary call to get_dhcp_port from DeviceManager

Closes-Bug: #1263217
Implements: rally-user-story US4051
Implements: rally-task TA3466
Upstream-Review: https://review.openstack.org/#/c/63492/
Not-in-upstream: true
Merge pull request #12 from CiscoSystems/us4051

Remove unnecessary call to get_dhcp_port from DeviceManager