Fixed incorrect include of "common" headers in mesos-execute.

Review: https://reviews.apache.org/r/18667
Updated dash in front of flags for tar command in fetcher.

Review: https://reviews.apache.org/r/18697/
Changed SIGTERM to SIGKILL in command executor.

Reverting signal from SIGTERM to SIGKILL again per discussion in
https://reviews.apache.org/r/18604/.

Review: https://reviews.apache.org/r/18661