Managing a new system user and group graylog2 in graylog2-server.spec

At the pre stage the system group graylog2 is created if it doesn't already exist, same with the
user.
At the preun stage this user and group are removed if they aren't already, using getent.

This new user/group will maintain ownership of the log directory, as write permissions are required,
and the install root /opt/graylog2-server/
Maintaining the new system user and group graylog2 in graylog2-web-interface.spec

In the pre stage the graylog2 group is created, if it hasn't already (using getent), then the
user is created, if it hasn't already.

The graylog2 user/group will be the new owner of the log directory and everything under /opt/graylog2-web-interface

CONCERN: If this package is installed on the same server as the graylog2-server package and one of these
packages is removed the other may not work properly. May need to consider combining the spec files.
At the preun stage this new user and group is removed.
Merging the last 2 change log entries to a single one in graylog2-server.spec