The symptom is that a route "fails" to create, then every subsequent
`terraform apply` fails with RouteAlreadyExists.
CreateRoute was succeeding but the very next DescribeRouteTables
was not listing the new route.
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 Stack