Support configuring an OpenID provider for authentication

[Free Ekanayaka]

In order to be able to deploy our charm for Landscape staging/production, we need to support configuring authentication against on OpenID provider. This is done via a snippet like:

[landscape]
# ...
openid-provider-url = https://login.ubuntu.com/
openid-logout-url = https://login.ubuntu.com/+logout

in service.conf (see configs/production/service.conf).

Since LDS supports other authentication mechanisms too (e.g. pam) , perhaps we want to have a generic "authentication-system" key in our config.yaml with a default set to "password", but that can be configured with strings like "openid provider-url=https://login.ubuntu.com/ logout-url=https://login.ubuntu.com/+logout".