Fix regression in EC2 credential provider parsing

The only parser basically had::

    config = dict(line.split('=', 1) for line in lines if '=' in line)
    access_key = config.get('AWSAccessKeyId')
    secret_key = config.get('AWSSecretKey')

Which ignores lines that don't have a '=' char in it.
This behavior has been re-added and additional tests to cover a few of
these edge cases have also been added.
Merge branch 'ec2-cred-provider' into develop

* ec2-cred-provider:
  Fix regression in EC2 credential provider parsing