[SPARK-17062][MESOS] add conf option to mesos dispatcher

Adds --conf option to set spark configuration properties in mesos dispacther.
Properties provided with --conf take precedence over properties within the properties file.
The reason for this PR is that for simple configuration or testing purposes we need to provide a property file (ideally a shared one for a cluster) even if we just provide a single property.

Manually tested.

Author: Stavros Kontopoulos <st.kontopoulos@gmail.com>
Author: Stavros Kontopoulos <stavros.kontopoulos@lightbend.com>

Closes #14650 from skonto/dipatcher_conf.