Ensure ipa-adtrust-install is run with Kerberos ticket for admin user
    
    When setting up AD trusts support, ipa-adtrust-install utility
    needs to be run as:
       - root, for performing Samba configuration and using LDAPI/autobind
       - kinit-ed IPA admin user, to ensure proper ACIs are granted to
         fetch keytab
    
    As result, we can get rid of Directory Manager credentials in ipa-adtrust-install
    
    https://fedorahosted.org/freeipa/ticket/2815