Use token and discovery fixture in identity tests

Replace a bunch of copied token data by using the fixtures available in
client.

Change-Id: Iec2e75cb4ffe9cec0396e9d8ec21a5e3c6816af7
Add docs for how to create an OAuth auth instance

Similar to the other authentication methods that were documented,
like password and token, this patch will add a brief description
on how to authenticate with the oauth1 plugin.

Change-Id: I1b7cceb116d337be71576139601f34ca17563a50
Merge "Use token and discovery fixture in identity tests"
Merge "Add docs for how to create an OAuth auth instance"