test/api: add checkEnvVSphere() to common/vsphere.sh

Add a common function to check for all environment variables needed to
test VSphere image. Call the function from all S3 implementations of
`checkEnv()` if testing the VSphere image type.

test/cases/api/generic.s3.sh

@@ -7,6 +7,9 @@ source /usr/libexec/tests/osbuild-composer/shared_lib.sh
 
 function checkEnv() {
     printenv AWS_REGION > /dev/null
+    if [ "${IMAGE_TYPE}" == "${IMAGE_TYPE_VSPHERE}" ]; then
+        checkEnvVSphere
+    fi
 }
 
 # Global var for ostree ref