Fix cert detection with custom PYTHON3_VERSION

PYTHON3_VERSION was not considered for detecting
ca path with GLOBAL_VENV=False, this patch fixes it.

Related-Bug: #2109591
Change-Id: Ie597494a2a11293cbd20e6d0b23dcb31bf343957

Author: karelyatin

lib/tls

@@ -367,7 +367,7 @@ function fix_system_ca_bundle_path {
         if [[ "$GLOBAL_VENV" == "True" ]] ; then
             capath=$($DEVSTACK_VENV/bin/python3 -c $'try:\n from requests import certs\n print (certs.where())\nexcept ImportError: pass')
         else
-            capath=$(python3 -c $'try:\n from requests import certs\n print (certs.where())\nexcept ImportError: pass')
+            capath=$(python$PYTHON3_VERSION -c $'try:\n from requests import certs\n print (certs.where())\nexcept ImportError: pass')
         fi
         if [[ ! $capath == "" && ! $capath =~ ^/etc/.* && ! -L $capath ]]; then
             if is_fedora; then