Add more indices
    
    ipaCASubjectDN is used by lightweight sub CA feature.
    
    ipaExternalMember is used by KRB driver to assemble MS-PAC records.
    
    ipaNTSecurityIdentifier was only index for "pres" and was missing an
    index on "eq". Samba and ipasam perform queries with SID string.
    
    memberPrincipal is used by S4U2Proxy constrained delegation and by
    ipa-custodia.
    
    Also note that dnaHostname, ipServiceProtocol, ipaCertSubject, and
    ipaKeyUsage are currently not index because an index would rarely used
    or have a poor selectivity.
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>