Add a new supported key to the config which allows the path to a
credentials file to be passed through to the ObjectStore plugin.
If that key is set in the config, use that file as the credentials
file for the AWS client.

This can be tested by adding a new key to the `cloud-credentials` secret
and adding the key value pair `credentialsFile=/credentials/<key_name>`.

Signed-off-by: Bridget McErlean <bmcerlean@vmware.com>

We have some repeated logic for creating session option. This introduces
a new function to handle the creation of those session options.

Signed-off-by: Bridget McErlean <bmcerlean@vmware.com>

Signed-off-by: Bridget McErlean <bmcerlean@vmware.com>

Signed-off-by: Bridget McErlean <bmcerlean@vmware.com>