Permission changes for dev/check/$Suite for aws module checks

Relies on https://github.com/silverstripe-labs/silverstripe-environmentcheck/pull/15

See merge request !1

Additionally, cache credentials if the credentials were retrived from
the metadata server via EC2. This improves performance.

This determines when the session should be garbage collected due to
inactivity which GarbageCollectSessionCronTask can do, if the
silverstripe-crontask module has been setup on your instance.

Use new class DynamoDbSession for registering and collecting the
configured database for sessions.