The version of PyYAML vendored in gpMgmt/bin/ext is old, unmaintained,
and does not support python3. Actually, it does not even contain a
`__version__` attribute, so it is not possible to know the version.

We need to unvendor yaml and address CVEs that have been found in the
library since the version vendored in source.

Also update yaml.load to use yaml.safe_load instead.
Co-authored-by: NTyler Ramer <tramer@vmware.com>
Co-authored-by: NJamie McAtamney <jmcatamney@vmware.com>

of configure