Support passing multiple Marathon servers to MarathonClient()

Summary:
Servers are tried in order until a non-"Connection Error" type response is received

Also included:
- Convert double-quoted strings to single-quoted

Test Plan: Test with a combination of live and dead URLs

Reviewers: merkkila, tberman

Reviewed By: merkkila, tberman

Subscribers: tberman, peter, justin, stephan, serdar

Differential Revision: http://phabricator.thefactory.com/D3434