Use modern GPL license tag

refs #5818
Logrotate.conf doesn't need execution rights

refs #5818
Add log directory to icinga2-common

RPMLint complains if logrotate is used without including the log directory in the package.

refs #5818
Don't copy INSTALL documentation to system docs

The INSTALL documentation is not relevant after installation, especially
if packages are used.

This does prevent a rpmlint warning.

refs #5818
Recommend logrotate in icinga2-common

Since we install a logrotate configuration file it does make sense to
at least recommend the installation of logrotate.
This addition is restricted to SUSE distributions since "Recommends"
is not supported elsewhere.

refs #5818
Description should be longer than the summary

This fixes a rpmlint warning.

refs #5818
Remove .gitignore files from package

This commit can be reversed as soon as there are upgrade files in the
schema upgrade directories.

refs #5818
Activate systemd on SUSE distributions

refs #5818
Reduce redunancies in spec file

Remove redundant differences between SUSE and RHEL distributions.

refs #5818
Remove redundant mysql requirement

The mysql dependencies are recognized automatically during building.

refs #5818
Remove obsolete pgsqllib require on SUSE distributions

refs #5818
Use root as default user and group to prevent issues

refs #5818
Use permission verfication scripts on SUSE distributions

refs #5818
Prevent empty icinga2 package by adding COPYING

Empty packages are not allowed on SLES11 and also produce a warning in
newer openSUSE releases.

So we just work around the issue by adding the COPYING file.

refs #5818
Use Github url as tarball source

This allows the automatic download of new versions.

refs #5818
Remove redundant RPM Requires

refs #5818
Merge branch 'fix/gnb-adapt-spec-for-suse-5818'

fixes #5818