cli: job create: fix --env and -r/--register

nargs causes argparse4j to consume flags and arguments following --env
and -r/--register. E.g. -r foo/tcp=http -p http=80 would cause -p and
http=80 to become appended as -r arguments into the registration list.

nargs is not useful here, so simply remove it.
Merge pull request #73 from spotify/dano/cli-fix-job-create-env-and-registration

cli: job create: fix --env and -r/--register