Switch credentials to use a AWSCredentialsProvider.

This is step 1 of moving to Instance Profiles where we will not need to manually manage keys.

http://docs.aws.amazon.com/IAM/latest/UserGuide/instance-profiles.html
Switch to using a credentials provider chain. The credentials in the properties file are now a fallback
Cleaning up some imports
Merge branch 'master' of github.com:guardian/frontend into credentials
Merge branch 'master' of github.com:guardian/frontend into credentials
Merge branch 'master' of github.com:guardian/frontend into credentials
Merge branch 'credentials' of github.com:guardian/frontend into credentials
Remove the InstanceProfileProvider for now.

We don't want it picking up incomplete credentials while we are busy fiuguring it out
Merge pull request #2635 from guardian/credentials

Switch credentials to use an AWSCredentialsProvider.