The Go coverage tool does not currently support recording a coverage
data profile across packages, so we must manually combine these coverage
profiles and use it to produce an HTML report when KUBE_COVER is
nonempty. The exact value of KUBE_COVER is now ignored; KUBE_COVERMODE
can be used to set the coverage mode from the default of "atomic".

Additionally, if KUBE_GOVERALLS_BIN is set, hack/test-go.sh will attempt
to report coverage results to Coveralls.io. This is intended to be used
with the Travis build.