provider/ec2: set UploadArches for tests

Ensure we upload AMD64 tools in tests,
regardless of host architecture.

Fixes https://bugs.launchpad.net/juju-core/+bug/1372961
Add comment about arch coverage
Merge pull request #822 from axw/lp1372961-ec2-tests-i386

provider/ec2: set UploadArches for tests

Ensure we upload AMD64 tools in tests,
regardless of host architecture.

Fixes https://bugs.launchpad.net/juju-core/+bug/1372961