cainstance.is_crlgen_enabled: handle missing ipa-pki-proxy.conf
    
    A failed ipa-ca-install left my installation in an inconsistent
    state.  Then, 'ipa-server-install --uninstall' also failed when
    is_crlgen_enabled() tried to read ipa-pki-proxy.conf, which was
    missing.
    
    Update is_crlgen_enabled() to handle missing ipa-pki-proxy.conf, by
    raising InconsistentCRLGenConfigException instead of RuntimeError.
    As a result, missing ipa-pki-proxy.conf is handled gracefully
    because the calling code already catches
    InconsistentCRLGenConfigException.
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>