Select LDAP config by extracted attribute

[shaardie]

This patch introduces a new global config variable provider_attribute to make it possible to select the config not only by entity but also select the config variable by a previous set attribute. This way it is possible to use a single point of authentication, but enrich the information from different ldap server based on e.g. the domain attribute extracted in previous steps.

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[shaardie]

Any update here? Is something missing in this PR for approval?