aws.py: check that subnet ID really exists
aws.py: check that subnet ID really exists

Also clean up by moving VPC validation code into a function.

Signed-off-by: Nathan Cutler <f9d8c66a4e898e67db786769b5b347b640e1a9a6@suse.com>

smithfarm/ceph-auto-aws,smithfarm/ceph-auto-aws