Allow configurable metadata for Splunk log driver
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 <e7d361c7f6c045f0c4fa54d2afc4a237f30bc767@gladkikh.email>

e11137/cli,e11137/cli,ashfall/cli,cpuguy83/cli,riyazdf/cli,docker/cli,dnephin/cli,thaJeztah/cli,docker/cli,allencloud/cli,ashfall/cli,redpanda/cli,allencloud/cli,dnephin/cli,Zebrilee/cli,cpuguy83/cli,redpanda/cli,riyazdf/cli,thaJeztah/cli,Zebrilee/cli