broadcast: settings in file

[matthewrmshin]

While the --set=[SEC]ITEM=VALUE option to broadcast is good enough for most cases, it can be quite difficult for:

• multi-line values
• values with lots of shell meta characters.
• many values

It should be possible to support broadcast options from a file that looks like a suite.rc fragment (with 2 section levels removed?)

Either:

[runtime]
    [[__BROADCAST__]]
        script = printenv FOOBAR
        [[[environment]]]
            FOOBAR=foobar

Or:

# Broadcast
script = printenv FOOBAR
[environment]
    FOOBAR=foobar

We'll then do cylc broadcast SUITE --file=FILE?

[hjoliver]

Good idea.