extending the default syslog logging configuration

### Nomad version
v0.5.2

### Operating system and Environment details
Debian 8

### Issue
I would like to keep the logging driver syslog with the syslog server managed by nomad but add certain config options (tag, env, ... )

```
config {
logging {
type = "syslog"
config {
tag = "foo"
}
}
}
```
Inspecting the running docker container shows that the syslog address is gone

```
"LogConfig": {
"Type": "syslog",
"Config": {
"tag": "foo"
}
}
```

Having the default syslog with additional config is great because ```nomad logs``` would still work I guess, as well as log rotation and cleanup.

### Reproduction steps
Just add the logging snipped to a job, run it and inspect the container.

Start with the job's logging configuration and inspect the resulting Docker container's LogConfig, focusing on how the syslog address and user-supplied options are represented. Reproduce the issue with the provided job snippet; done means the default syslog address remains available alongside options such as tag, while nomad logs, rotation, and cleanup continue to work.

Written by the indexing model from the issue text.