Cleanup rename of get_compute_ports_on_host_by_subnet

A recent merge [1] renamed this method to remove "compute_" from the
name.  Unfortunately, the rename was not done thoroughly.  This patch
cleans that up.  No bug is referenced since this is an obvious
ommission in a recent patch.

[1] https://review.openstack.org/#/c/114141/

Change-Id: I8320bc8deb6dcda586ccff01120270b3ce257ea2
Merge "Cleanup rename of get_compute_ports_on_host_by_subnet"