Add the --user option and improve command options.
Add the --user option and improve command options.

This change adds the --user option to specify the remote user for ssh,
removes the --template option in favor of a universal --command option
and renames options for simpler usage.

In particular, the following options changed:

Added:   --user
Removed: --template
Renamed: --cmd     to --script
Renamed: --cmdline to --command

m-lab/operator,stephen-soltesz/operator,nkinkade/operator,salarcon215/operator,salarcon215/operator,critzo/operator,critzo/operator,stephen-soltesz/operator,nkinkade/operator,jheretic/operator,jheretic/operator,m-lab/operator