Fix 3.8 and 3.9 wheels for linux x86_64

Actually use manylinux2014 docker image. (#194)

Actually use the manylinux2014 docker images, instead of just running on an EC2 instance that's similar enough that we happen to pass the wheel audit.

Also, setup.py improvements:
- accept `cmake3` as well as `cmake`.
- slight cleanup of how AWS_LIBCRYPTO_INSTALL is used.