Support alternate MMS API URLs

MMS exists both as a hosted service as well as a suite of applications
that users can install within their own infrastructure. Users with large
clusters of MongoDB instances to monitor and backup will generally
prefer On-Premises MMS due to the savings in time/resources for
transferring backups as well as the costs savings from not paying
MongoDB for backup storage.

This small patch adds support for setting a custom MMS API URL to allow
the tool to be used against mms services other than the official
mongodb.com service.

(p.s. Ruby is not one of my strong languages. All pointers/criticism greatly appreciated)