add support for multiple LDAP/AD servers for failover

this is a request to add support to the configuration for multiple AD/LDAP servers to allow for failover/redundancy when a AD server is unavailable
something like
```
 38 auth:
 39   ldap:
 40     type: ldap
 41     client_options:
 42       url: "ldaps://ad02.local.net:636 ldaps://ad01.local.net:636"
```