Change [nova_client]/api_version default to service_default

With [1] current default '2' for [nova_client]/api_version
no longer works, it needs version to be in format X.Y,
let's use watcher service default which is 2.56 currently.

[1] https://review.opendev.org/#/c/659194/
Change-Id: Ieea49092c5f323686844161361a6e5e24e4971f6

manifests/init.pp

@@ -26,7 +26,7 @@
 #
 # [*nova_client_api_version*]
 #   (required) Version of Nova API to use in novaclient.
-#   Default is 2.
+#   Default is $::os_service_default
 #
 # [*package_ensure*]
 #  (optional) Whether the watcher api package will be installed
@@ -304,7 +304,7 @@ class watcher (
   $cinder_client_api_version            = '2',
   $glance_client_api_version            = '2',
   $neutron_client_api_version           = '2',
-  $nova_client_api_version              = '2',
+  $nova_client_api_version              = $::os_service_default,
   $package_ensure                       = 'present',
   $rabbit_login_method                  = $::os_service_default,
   $rabbit_retry_interval                = $::os_service_default,

releasenotes/notes/change_nova_api_version_default-f372441622a9ebd2.yaml

@@ -0,0 +1,6 @@
+---
+other:
+  - |
+    Integer based api_version is no longer supported since
+    https://review.opendev.org/#/c/659194/. It's good to rely
+    on watcher service default to configure [nova_client]/api_version.