Make credentials system in Riff-Raff generic
Improvements to previous implementation

 - change deployinfo key to be consistent with property name
 - use ApiCredential class more widely
 - move to map (keyed by service) instead of list
 - verbose log entire keyring rather than on AWS call
Merge pull request #110 from guardian/sh-make-credentials-sane

Make credentials system in Riff-Raff generic