Improve @base.scenario to specify it is admin only

Adds extra field "admin_only" to mark all benchmark scenarios
that require admin endpoints. "admin_only" is set to False by
default, but for scenarios that require admin endpoints
should use @base.scenario(admin_only=True)

blueprint: benchmark-context-predefined-users
Change-Id: I5f6a6b7a41dac1c08f7e084e7eee6ca83bd3b6d9
Merge "Improve @base.scenario to specify it is admin only"