Updating both packages to 0.20.0-preview.7 in SPECS/graylog2-server.spec and SPECS/graylog2-web-interface.spec

The latest version of the graylog2-server and graylog2-web-interface packages have been released
earlier this week, updating SPEC files for new version.

Changing the path for the graylog2_conf file from /etc/graylog2.conf to /etc/graylog2/graylog2.conf
in SOURCES/init.d-graylog2-server and SPECS/graylog2-server.spec. This will add more flexibility
in the future as it will be easier to customize config file locations.
Updating files list in SPECS/graylog2-server.spec to put the graylog2.conf file in the proper location.

The location of the graylog2.conf file was changed from /etc/ to /etc/graylog2/, the file list in the
SPEC file was missed when the initial update was committed.

Updating the changelog to mention that the configuration file location was changed.
Setting the default classpath variable in init.d-graylog2-web-interface to $lib_dir/*
Setting the backup classpath varialbe in sysconfig-graylog2-web-interface to a output from ls
and setting the graylog2-web-interface jar version from preview.6 to preview.7