aws-sso should support AWS GovCloud start URLs

[jmontroy90]

Is your feature request related to a problem? Please describe.
I'm unable to use aws-sso with AWS GovCloud. The access portal URL format is different for GovCloud, as seen on the third bullet point here: https://docs.aws.amazon.com/govcloud-us/latest/UserGuide/govcloud-sso.html

Describe the solution you'd like
Allow for Govcloud-formatted start URLs.

Additional context

[jmontroy90]

Presumably it's something in promptStartUrl using the constants:

START_URL_FORMAT       = "https://%s/start"
START_FQDN_SUFFIX      = ".awsapps.com"