[Fixes #51] Allow failover tests to be skipped

Add a new `-skipFailover` flag that will prevent the additional "backup"
backends from being setup and skips all tests in cdn_failover_test.

I did start an implementation a while ago whereby you could specify a
variable number of backends, but I don't think it's that useful right now.
This is the most efficient thing to do.
Merge pull request #73 from alphagov/51-skip_failover_tests

[Fixes #51] Allow failover tests to be skipped