Fedora 30 : add python-path for Python 3

It appears there are some issues with Ansible + python2 + Fedora 30,
particularly around package installation (e.g. [1]).

Set the python-path to Python 3 for Fedora 30

[1] https://github.com/ansible/ansible/issues/54855

Change-Id: Ic9027d41af8d29d2c6a33cc1a0c57204899741d2

nodepool/nodepool.yaml

@@ -247,6 +247,7 @@ diskimages:
       DIB_SHOW_IMAGE_USAGE: '1'
   - name: fedora-30
     pause: false
+    python-path: /usr/bin/python3
     elements:
       - fedora-minimal
       - vm