Enable logging configuration via config file

[pettermahlen]

This PR adds (yet) another SyslogAppender subclass that replicates some of the configuration
setup done by LoggingConfigurator. The idea is to make it possible to easily use a regular logback.xml file and not have to use the programmatic API exposed by LoggingConfigurator.

An example logback.xml file illustrating usage:

<configuration>

    <appender name="syslog" class="com.spotify.logging.logback.EnvironmentVariableSyslogAppender">
        <serviceName>acoolservice</serviceName>
    </appender>

   <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
        </encoder>
    </appender>

    <root level="DEBUG">
        <appender-ref ref="syslog" />
        <appender-ref ref="STDOUT" />
    </root>
</configuration>

[pettermahlen]

@jo-ri

[jo-ri]

shouldAddServiceNameToSuffixPattern?

[jo-ri]

+1