Added LDAP support

Added LDAP support. Disabled by default, add the necessary configuration in inventory or some other suitable place to enable:
```
graphite_ldap_enabled: True
graphite_ldaps_enabled: False
graphite_ldap_host: "ldap.mycompany.com"
graphite_ldap_port: 389
graphite_ldaps_port: 636
graphite_ldap_search_base: "OU=users,DC=mycompany,DC=com"
graphite_ldap_base_user: "CN=some_readonly_account,DC=mycompany,DC=com"
graphite_ldap_base_pass: "readonly_account_password"
graphite_ldap_user_query: "(username=%s)"

```