Don't upgrade the schema on frontends

Upgrading from EC 11.2.5 to 12rc6 on a FE yields this error

```
ok: down: nginx: 1956s, normally up 
ok: down: oc_bifrost: 1956s, normally up 
ok: down: oc_id: 11256s, normally up 
ok: down: opscode-account: 11255s, normally up 
ok: down: opscode-certificate: 11254s, normally up 
ok: down: opscode-erchef: 1955s, normally up 
ok: down: opscode-reporting: 11253s, normally up 
[private-chef-upgrade] - Latest Migration Available: 1.19 
[private-chef-upgrade] - Migrations to Run: [1.10, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19] 
[private-chef-upgrade] - Current Migration Version: 1.9 
[private-chef-upgrade] - Starting Migration 1.10 
[private-chef-upgrade] - Starting Service postgresql 
[private-chef-upgrade] - Private Chef Role: frontend 
[private-chef-upgrade] - Skipping Schema Upgrade 
[private-chef-upgrade] - Stopping Service postgresql 
[private-chef-upgrade] - Error: service postgresql failed to stop, killing gracefully... 
[private-chef-upgrade] - Sleeping for 10 seconds while service postgresql stops... 
[private-chef-upgrade] - Failure: service postgresql could not be stopped or killed. 
```