[r=jameinel],[bug=1290841] cmd/juju/machine: Enable GOMAXPROCS(NumCPU)
As part of my scale testing work, I noticed the API server is CPU bound
in many circumstances (logging in lots of machines at the same time).
This changes it so machine agents that manage the environment (i.e. the
API Server) changes the default for GOMAXPROCS to the number of CPUs.