Update Nova release liaisons

Sean wanted to stop being the release liaison so eventually René and Amit
agreed on helping.
Amit will be the main liaison for stable branches while René will be helping
for libraries, placement and master branch of the nova repo.

Change-Id: I4d43469582d9745111105caf63d6709f02725e1b

data/release_liaisons.yaml

@@ -85,9 +85,12 @@ neutron:
     irc: slaweq
     email: skaplons@redhat.com
 nova:
-  - name: 'Sean Mooney'
-    irc: sean-k-mooney
-    email: smooney@redhat.com
+  - name: 'René Ribaud'
+    irc: Uggla
+    email: rene.ribaud@gmail.com
+  - name: 'Amit Uniyal'
+    irc: auniyal
+    email: auniyal@redhat.com
 octavia:
   - name: 'Michael Johnson'
     irc: johnsom