Standardize MAAS provider's imports on new convention, and ditch use of new() to create gc.Suite.
[r=jtv] Standardize MAAS provider's imports.

This applies the new import conventions consistently throughout the MAAS
provider:

 1. Separate sections for standard-library, non-standard, and in-project
 imports.

 2. Import gocheck as "gc."

In addition to this, I made the tests create their Suite object consistently
using &Struct{} syntax. John did not like our use of new(), and so we stopped
doing it but never cleaned out the old usage.

I'm hoping for a "trivial" approval for this, because keeping these branches
in review for too long just causes unnecessary conflicts. Also, I'd like to
follow up with a branch that un-exports some items that do not need to be
public. I did the same for the Azure provider.