[Alerting] default message for index threshold includes both threshold values even if not used

[pmuellr]

I'm seeing the following message get generated using the {{{context.message}}} variable from the index threshold alert:

alert cpu usage group host-C value 50 exceeded threshold count < 100,5000 over 50s on 2020-03-12T17:35:25.892Z

Note the comparator here - < - only needs the first threshold value, but both are printed - 100,5000. It should just be printing 100.

It's probably easiest to fix this in the alertType message variable generator, as it's kind of nice to persist that 2nd value if the customer switches between a comparator that just needs one value (most of them) and one that needs two (between, notbetween).

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)