[test] Sanity-check CLOUD_NAME value

The OpenShift::UnixUser class interpolates the value of CLOUD_NAME in
/etc/openshift/node.conf into the name of a shell environment variable.
Therefore, if CLOUD_NAME contains symbols that may not appear in a shell
variable name, we will get mysterious errors.

This patch adds a check in the initializer for OpenShift::UnixUser to
verify that CLOUD_NAME contains only valid symbols.