Make the 'admin' role configurable

Make the 'admin' role configurable

Bug 1158434

This patch adds a new policy named 'context_is_admin' which defines
an admin user as a collection of roles or else. The quantum context
has been updated to check for this policy when setting the is_admin
flag.
This patch also adds a method for gathering 'admin' roles from policy
rules as current logic requires the context to be always populate with
the correct roles for admin rules, even when the context is implicitly
generated with get_admin_context or context.elevated.
Backward compatibility is ensuring by preserving the old behavior if
the 'context_is_admin' policy is not found in policy.json

Change-Id: I9acea75cca0c47e083a9149e358328ea3ca12d68