Makefile: add variable for extra go flags in gotest
Makefile: add variable for extra go flags in gotest

Will allow to run race detector in Travis.

csrwng/go-dockerclient,ybogdanov/go-dockerclient,tarsisazevedo/go-dockerclient,davidstack/go-dockerclient,paulbellamy/go-dockerclient,fsouza/go-dockerclient,davidstack/go-dockerclient,tarsisazevedo/go-dockerclient,ybogdanov/go-dockerclient,csrwng/go-dockerclient