Add admin_auth_token option for nova driver

The call to nova.context.get_admin_context() never sets an auth_token,
but we should support the idea of having an admin token that's always
known to be good. This adds a new Ironic config option for
'admin_auth_token' and uses it if set.

DocImpact

Closes-bug: 1292676

Change-Id: I73fcf13e2a746be527c234bdc711d95de157a9e5
Merge "Add admin_auth_token option for nova driver"