Bug 19185: Use submit button if only one exists on the page

Bug 19185: Use submit button if only one exists on the page

During the installation process we do not have fieldset.action, in order
to not complicate changes we are just going to use the submit button if
only one exists.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>