Halve the number of cylc python processes with exec.

Uses exec to replace the main cylc command process with the sub-command,
instead of running the sub-command as a sub-process of the main command.
Removed some cruft.
Removed some more cruft.
Merge pull request #219 from hjoliver/use-exec-in-main-cli

Halve the number of cylc python processes with exec.