[3689] Removed call to findReservation from AllocateEngine::allocateLeases6()

[3689] Removed call to findReservation from AllocateEngine::allocateLeases6()

src/lib/dhcpsrv/alloc_engine.h
src/lib/dhcpsrv/alloc_engine.cc
    allocateLeases6()
        - removed find_reservation parameter from allocateLeases6
        - removed logic that called findReservation(), this eliminates
        inadvertanly stomping an already populated host in the context.
        Places burden of doing the reservation lookup always on the caller.

src/bin/dhcp6/dhcp6_srv.cc
    removed obsolete boolean parameter from allocateLeases6 calls

src/lib/dhcpsrv/tests/alloc_engine_utils.h
src/lib/dhcpsrv/tests/alloc_engine_utils.cc
   AllocEngine6Test::findReservation() - new method which calls
   engine's findReservation() and fills in context hostname accordingly.

src/lib/dhcpsrv/tests/alloc_engine6_unittest.cc
    added calls to AlloceEnginetTest::findReservation where needed