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.
Recommend logrotate in icinga2-common

Since we install a logrotate configuration file it does make sense to at
least recommend the installation of logrotate.
Excluding this addition to SUSE distributions since Recommends is not
supported elsewhere.
Description should be longer than the summary

This fixes a rpmlint warning.
Remove .gitignore files from package

This commit can be reversed as soon as there are upgrade files in the
schema upgrade directories.
Remove redundant requirements for SUSE distributions
Activate systemd on SUSE distributions
Reduce redunancies in spec file

Remove redundant differences between SUSE and RHEL distributions.
Remove redundant mysql requirement

The mysql dependencies are recognized automatically during building.
Remove obsolete pgsqllib require on SUSE distributions
Use root as default user and group to prevent issues
Use permission verfication scripts on SUSE distributions
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.