Google OpenID Support

[jjaraalm]

Adds support for authenticating against Google OpenID and potentially other providers. Client support is in HDFGroup/h5pyd#81.

Azure tests should be repeated (I do not have Azure access) and docs need to be updated still.

New settings

1. openid_provider - provider to use for OpenID authentication. Valid values are azure (default) and google.
2. openid_audience - The intended audience of the OpenID token. If not set, falls back on the azure_resource_id variable. For Google, this should be the client_id.
3. openid_claims - List of claims to use when resolving usernames from OpenID token in order of priority. The default is the previously hardcoded Azure setting unique_name,appid. Neither of these are defined by Google. sub would be valid for all OpenID configurations, but is opaque and not helpful for setting ACLs.

[jreadey]

Thanks @jjaraalm! This was working for me with AD. I made one change: 6ad25d3 since sometimes the config get evaluates to empty string vs None.