Adding etcd username on linux:system level

The patch moves creation of etcd username on linux:system to be
created by linux.system state because of the fact that in salt > 2016.11
behavior of gid_from_name parameter has been changed so currently if
gid_from_name = True and the group doesn't exist then etcd.server.service
will fail.

https://docs.saltstack.com/en/2016.3/ref/states/all/salt.states.user.html
https://docs.saltstack.com/en/2017.7/ref/states/all/salt.states.user.html

Change-Id: Iea3f9811b83ec9239ce2c219ff3ad99fd6cf2ac3
Related-PROD: PROD-20579
Related-PROD: PROD-20581
diff --git a/metadata/service/server/proxy.yml b/metadata/service/server/proxy.yml
index 628bdc7..02e77dd 100644
--- a/metadata/service/server/proxy.yml
+++ b/metadata/service/server/proxy.yml
@@ -2,6 +2,7 @@
 - etcd
 classes:
 - service.etcd.support
+- service.etcd.linux
 parameters:
   etcd:
     server:
@@ -15,4 +16,4 @@
         - host: ${_param:cluster_node02_address}
           name: ${_param:cluster_node02_hostname}
         - host: ${_param:cluster_node03_address}
-          name: ${_param:cluster_node03_hostname}
\ No newline at end of file
+          name: ${_param:cluster_node03_hostname}