Move shared settings from aws (ec2 and s3 plugins) to plugin specific settings

[rjernst]

We currently share configuration settings for discovery-ec2 and repository-s3, like access and secret keys. I don't this gains that much in less configuration for a user, and it was suggested in #22762 (comment) that we move these shared settings to plugin specific.

[rjernst]

These shared settings are now removed in master.