Only enable LDAP providers if LDAP is enabled
Merge branch 'issue-ldap-initialization' into 'master'

Issue ldap initialization

If LDAP is disabled error will appear:

```
ERROR -- : Missing setting 'servers' in 'ldap' section in /Volumes/Work/gitlab/development/community/gitlab/config/gitlab.yml (Settingslogic::MissingSetting)
```

@marin Can you test if this will solve the startup problem?

See merge request !1179
Merge branch 'master' of github.com:gitlabhq/gitlabhq