remove the empty case for OpenBSD in package.pp installation, but enhance the default case, to allow installation of package also on OpenBSD.

reason for that is that OpenBSD recently dropped sudo from base
and moved it to ports/packages.
Made decision based on ::kernelversion, to default package
package name for OpenBSD, everything smaller than 5.7, won't
install a package, and all => 5.8 will install sudo package.
I did use kernelversion, since that is more stable, operatingsystemrelease
has -stable, -snapshot, -beta etc. suffixes, that cause trouble
with versioncmp().

In init.pp, had to make the config_file and config_file_dir
dependent on the package subclass, instead of the package
itself, since with 5.7, no package is installed that can be
referenced.

while there, I added/enhanced spec tests for OpenBSD package
installation.