makefile: use variables for go and gofmt commands
makefile: use variables for go and gofmt commands

There are use cases where the default go commands are not what you want,
including an upcoming change. This simply "variablizes" the makefile.

Signed-off-by: John Mulligan <eda8ad33169deab44f9fe65847f99e368da6c91b@redhat.com>

ceph/go-ceph,ceph/go-ceph,ceph/go-ceph