Testing the log4j configuration file with xmllint

https://github.com/corey-hammerton/graylog2-rpms/issues/5

Files Modified:
SOURCES/init.d-graylog2-server
SOURCES/sysconfig-graylog2-server
SPECS/graylog2-server.spec

Creating new variable GRAYLOG2_LOG_CONF in
SOURCES/sysconfig-graylog2-server in replacement for the
fixed string in the ADDITIONAL_JAVA_OPTS variable. This is the
only possible way to test the syntax of the log4j configuration.
Doing the same in SOURCES/init.d-graylog2-server

Checking for the existence of the GRAYLOG2_LOG_CONF variable in
SOURCES/init.d-graylog2-server before running xmllint on it in the
configtest function.

Adding a new RPM dependency in SPECS/graylog2-server.spec for
libxml2, the provider of /usr/bin/xmllint.