Removing part of circular dependency of ipalib in ipaplaform
    
    After commit cac3475, ipa-backup is broken due to circular
    dependencies. This fixes it, removing circular dependency
    of ipalib. The ipalib.constants.IPAAPI_USER is now passed
    as parameter to the function that use it.
    
    https://pagure.io/freeipa/issue/7108
    
    Reviewed-By: Tomas Krizek <tkrizek@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>