Allow prep commands to take a file path instead of suite name.

Closes #419.
Same for get-config command.
Handle commands with no suite arg (e.g. scan).
Better variable name.
Fix search command for file path arg.
Take cylc-suite-env write out of task jobfiles.
Finished deprecating CYLC_SUITE_REG_NAME
Create suite env parent dir if necessary.
Get proper PID for local and remote background jobs.
Merge pull request #425 from hjoliver/reimplement-cylc-suite-env

Reimplement cylc suite env file write
Merge pull request #424 from hjoliver/419.prep-by-file-path

419.prep by file path