Change the button to "Select" instead of sign when we only want to
select a certificate.

Change-Id: Ifa896e502c8680497d71912ddd1058ab0b71542b
Reviewed-on: https://gerrit.libreoffice.org/52823Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>