Add the ability to scan multiple regions for MSK clusters

[joshm91]

We'd like the ability to specify a list of AWS regions to scan for MSK clusters and have the results combined in a single targets file.

[soloradish]

I impl this feature in my forked repo. but I tweak alot to fit my use case.

https://github.com/soloradish/prometheus-msk-discovery