Autodetect container type

[Haw Loeung]

Hi,

I think the charm could be more clever and by default, automatically detect the container type based on the information provided by container credentials. I know container credentials is provided as a base64 string, but it should be easy enough to decode.

For example, if given AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, and AWS_REGION[1], then container type would be "s3". If given os_username, os_password, os_tenant_name/os_project_name, and os_auth_url[2] then "swift". If given just private_key[3], then Azure... and whatever for GCE[4].

[1]https://git.launchpad.net/container-log-archive-charm/tree/files/archive-to-s3.py#n35
[2]https://git.launchpad.net/container-log-archive-charm/tree/files/archive-to-swift.py#n36
[3]https://git.launchpad.net/container-log-archive-charm/tree/files/archive-to-azure.py
[4]https://git.launchpad.net/container-log-archive-charm/tree/files/archive-to-gce.py#n75