Be able to configure multiple service accounts for different purposes

[fredreichbier]

Right now, we use the service account specified in [service-account] for two purposes:

• If bind-service-account is set to true, a bind request that was authenticated successfully against privacyIDEA will result in a bind request using the service account credentials against the LDAP backend. This makes it possible to perform searches (if allow-searches is also true).
• The lookup user mapping strategy uses the service account credentials to look up usernames.

Maybe we should make it possible to use different service account credentials here?

[cornelinux]

If there are different applications connected to the LDAP proxy, this exspecially seems to make sense...