config: template variables specified to play should be loaded

[oliver-sanders]

Template variables can be provided to cylc play, they are then stored in the DB so they can be used with future restarts.

However, the cylc config command is currently unaware of this:

$ cylc install one
$ cylc config
$ cylc play one -s "FOO='bar'"
$ cylc config  # should load FOO='bar' from the workflow DB

Note that Cylc-Rose can also be used to specify template variables (at install time), these variables are store in a file which is loaded when cylc config is invoked.

Pull requests welcome!

[oliver-sanders]

Note: This ability to extract template vars from the run dir is also required by #3896

[oliver-sanders]

This might be a feature nicely covered with a Cylc plugin i.e. a new entry point for functions which extract options and stuff from the run dir.

[oliver-sanders]

@wxtim This is related to cylc vro #5121

[wxtim]

@wxtim This is related to cylc vro #5121

I think that I already have a proof of concept which is nearly ready for this, so I've assigned it to myself.