Configure logging level in portable Spark runner

[damccorm]

log4j:WARN No appenders could be found for logger (org.apache.beam.vendor.grpc.v1p13p1.io.netty.util.internal.logging.InternalLoggerFactory).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
Using Spark's default log4j profile: org/apache/spark/log4j-defaults.properties

[1] doesn't seem to have any effect.

[1] 

beam/runners/spark/job-server/build.gradle

Lines 80 to 81 in c9fb261

	if (project.hasProperty("logLevel"))
	jvmArgs += ["-Dorg.slf4j.simpleLogger.defaultLogLevel=${project.property('logLevel')}"]

Imported from Jira BEAM-7805. Original Jira may contain additional context.
Reported by: ibzib.