Allow use of ENV vars in configuration parameters

[Phil Winder]

Automatically expand any CLI/Env var config options if they contain env vars. For example, many installations don't install the mesos sandbox and /tmp/mesos. They use /var/lib/mesos. So to make the default logstash example more universal this should be allowed:

    "EXECUTOR_FILE_PATH": "$MESOS_WORK_DIR/slaves/*/frameworks/*/executors/*/runs/*/stdout, $MESOS_WORK_DIR/slaves/*/frameworks/*/executors/*/runs/*/stderr, $MESOS_LOG_DIR/*.INFO, $MESOS_LOG_DIR/*.WARNING"

Note the use of valid Mesos Env vars to define the work directory and the log directory. But this doesn't work. It passes the $VAR straight through to logstash.

[James H. Fisher]

This is one case where it would be more natural to use an actual Logstash config file instead of CLI params or environment variables. The config file could have a $MESOS_WORK_DIR free variable. See #109