Activator: add unit test for cmd/activator/main.go

[akyyy]

/area autoscale

/kind cleanup

Expected Behavior

1. Add unit test to main.go
   [Mustafa] Roundtripper should be simple to test with a fake client. Though you will need to change the code slightly and instead of using http.DefaultTransport, you will need to pass it as an argument.
   to the rountripper struct.
2. activator is now the only code piece that use glog. We need to migrate that to zap logger as well

[bsnchan]

As part of the unit test we should ensure we test HTTP POST in addition to HTTP GET

See: #1292

[tanzeeb]

/assign @bsnchan @tanzeeb