Add fleets property to run configurations and CLI

[un-def]

Closes: #1447

[peterschmidt85]

@un-def The property in the run configuration YAML file (and API) should be fleets and must accept an array. Is it implemented this way? Just want to confirm.

[un-def]

@peterschmidt85 it accepts both a name and a list of names. I chose the singular form because I think it will be mostly used with a single fleet

[peterschmidt85]

@peterschmidt85 it accepts both a name and a list of names. I chose the singular form because I think it will be mostly used with a single fleet

I strongly suggest to use plural - similar to regions and backends and always require a list.