The "find route in table" helper code was not properly handling routes
with no destination CIDR block - like vpc_endpoint routes - so if one of
those routes would come up before the target route in the loop, we'd get
a crash.
Fixes#6337
Fixing basic acceptance test.
Adding warning to website about mixed mode.
Adding exists to aws_route.
Adding acceptance test for changing destination_cidr_block.
Paul Hinze