Fix #7944 add check/test for localhost in ParseHost

Fixes #7944, adds a test case for `addr` set to `localhost:5000`.

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
it is a constant https://github.com/docker/docker/blob/master/api/common.go#L16