Enable all disk templates in unit-tests

After the many patches implementing the shared storage functionality
in iallocator, balances, etc., and after the patches preparing the
unit-tests to do the right thing for shared storage, we can finally
flip the bits that make instances have "random" disk templates as
opposed to just DRBD. Yes, the unit-tests still pass ☺️



We also need to enable all disk templates in the IPolicy (for
allocation/relocation).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>