Added '/create-volumes' tests with multiple roles.
Added '/create-volumes' tests with multiple roles.

Operators may create volumes for multiple roles in the same operation;
this patch adds tests to confirm correct behavior of authorization in
this case. The tests
`ReservationEndpointsTest.GoodReserveACLMultipleRoles` and
`ReservationEndpointsTest.BadReserveACLMultipleRoles` were added.

Review: https://reviews.apache.org/r/43778/

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