Remove deprecated legacy credentials provider

This commit removes the legacy credential provider. It's been
deprecated for over a full cycle which has been sufficient time for
users to migrate off of it. Keeping this around just causes more
confusion by having a 3rd path to configuring credentials in tempest.
The credential provider was officially marked as deprecated in change
Ia5132c5cb32355d6f26b8acdd92a0e55a2c19f41

Change-Id: I8c24cd17f643083dde71ab2bd2a38417c54aeccb
8 files changed