Fix validate_release() for Ubuntu flavours

quickget

@@ -73,10 +73,15 @@ function pretty_name() {
 
 function validate_release() {
   local DISPLAY_NAME=""
-  local RELEASE_GENERATOR="${1}"
+  local RELEASE_GENERATOR=""
   local RELEASES=""
 
   DISPLAY_NAME="$(pretty_name "${OS}")"
+  case ${OS} in
+    *ubuntu*) RELEASE_GENERATOR="releases_ubuntu";;
+    *) RELEASE_GENERATOR="${1}";;
+  esac
+
   RELEASES=$(${RELEASE_GENERATOR})
   if [[ "${RELEASES}" != *"${RELEASE}"* ]]; then
       echo -e "ERROR! ${DISPLAY_NAME} ${RELEASE} is not a supported release.\n"