Allow configurable metadata for Splunk log driver

Add support of `tag`, `env` and `labels` for Splunk logging driver.
Removed from message `containerId` as it is the same as `tag`.

Signed-off-by: Denis Gladkikh <denis@gladkikh.email>

contrib/completion/bash/docker

@@ -337,7 +337,7 @@ __docker_log_driver_options() {
 	local journald_options="env labels"
 	local json_file_options="env labels max-file max-size"
 	local syslog_options="syslog-address syslog-facility tag"
-	local splunk_options="splunk-caname splunk-capath splunk-index splunk-insecureskipverify splunk-source splunk-sourcetype splunk-token splunk-url"
+	local splunk_options="env labels splunk-caname splunk-capath splunk-index splunk-insecureskipverify splunk-source splunk-sourcetype splunk-token splunk-url tag"
 
 	local all_options="$fluentd_options $gelf_options $journald_options $json_file_options $syslog_options $splunk_options"