Add the ability to specify a HELIOS_MASTER environment variable

It will tell the CLI where to talk to to reach a helios cluster.
It can either be http://location_of_master:port or site://sitename.
If a destination is specified on the commandline, it overrides the
environment variable.
Merge pull request #74 from spotify/drewc/cfg-by-env-uri

Add the ability to specify a HELIOS_MASTER environment variable