ipa-custodia: use Dogtag's alias/pwdfile.txt
    
    /etc/pki/pki-tomcat/password.conf contains additional passwords like
    replicadb. ipa-custodia does not need these passwords.
    /etc/pki/pki-tomcat/alias/pwdfile.txt holds the passphrase for Tomcat's
    NSSDB. The file also simplifies implementation because it removes
    another temporary file.
    
    pwdfile.txt is created by CAInstance.create_certstore_passwdfile()
    
    Related: https://pagure.io/freeipa/issue/6888
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Simo Sorce <ssorce@redhat.com>