Fix the reading of the localhost MTU in the HTTP tests

Sometimes the localhost interface has a different name from "lo",
this fix adds a helper function to read the MTU value when the
interface name starts with "lo".
Improve the detection of localhost interfaces for the HTTP test suite
Use ct_helper for retrieving the loopack interface MTU
Merge branch 'fix/http-tests-get-mtu' of git://github.com/soundrop/cowboy