Update min supported service version for Yoga

Nova only supports compute services that are not older than version
N-1. So in Yoga the smallest supported nova compute version is Xena
and the smallest Xena service version is 57.

Change-Id: I709b3967c9e6e390016c003f9e5b33ae55708d28
Signed-off-by: Takashi Natsume <[email protected]>

Author: takanattie

nova/objects/service.py

@@ -218,10 +218,11 @@
 # This is used to raise an error at service startup if older than N-1 computes
 # are detected. Update this at the beginning of every release cycle to point to
 # the smallest service version that was added in N-1.
-OLDEST_SUPPORTED_SERVICE_VERSION = 'Wallaby'
+OLDEST_SUPPORTED_SERVICE_VERSION = 'Xena'
 SERVICE_VERSION_ALIASES = {
     'Victoria': 52,
     'Wallaby': 54,
+    'Xena': 57,
 }