stem: re-implement as Cylc install plugin

[oliver-sanders]

The rose stem command wraps cylc install which is a bit awkward.

• The bespoke command is an additional complication to working practices.
• Cylc commands like cylc validate and cylc graph may fail on the source due to missing template variables.
• It makes Stem seem like a system onto itself whereas it is really just setting a couple of template variables.

Consider re-implementing rose stem as a pure plugin so that the Stem working practices are the same as for any other Cylc workflow. This would require Stem workflows to use a flag file or load a Stem plugin in some way.

For example if we were to implement a template variable file for Cylc, a Stem workflow might look something like:

# cylc_template_variable_file.py

from cylc.rose.stem import get_stem_variables

def get_template_variables():
    # this line makes a workflow a `rose stem` workflow
    return get_stem_variables()

Otherwise we could use a flag file or configuration something like:

# rose-suite.conf

[stem]  # activate rose-stem
groups=foo bar baz  # define defaults

[template variables]
OPT_1=True
OPT_2=False