Added context to new agent processing calls.
Added context to new agent processing calls.

This change adds context information to log entries for agent
HTTP API handlers. It impacts the calls `LAUNCH_CONTAINER`,
`WAIT_CONTAINER`, `KILL_CONTAINER`, and `REMOVE_CONTAINER`.

Review: https://reviews.apache.org/r/63833/

andschwa/mesos,gsantovena/mesos,chhsia0/mesos,Gilbert88/mesos,Gilbert88/mesos,reneploetz/mesos,andschwa/mesos,gsantovena/mesos,gsantovena/mesos,chhsia0/mesos,chhsia0/mesos,asamerh4/mesos,asamerh4/mesos,jpeach/mesos,asamerh4/mesos,reneploetz/mesos,gsantovena/mesos,Gilbert88/mesos,chhsia0/mesos,asamerh4/mesos,kaysoky/mesos,dforsyth/mesos,andschwa/mesos,andschwa/mesos,kaysoky/mesos,Gilbert88/mesos,jpeach/mesos,Gilbert88/mesos,abudnik/mesos,jpeach/mesos,jpeach/mesos,dforsyth/mesos,dforsyth/mesos,abudnik/mesos,gsantovena/mesos,kaysoky/mesos,jpeach/mesos,Gilbert88/mesos,abudnik/mesos,andschwa/mesos,andschwa/mesos,andschwa/mesos,abudnik/mesos,reneploetz/mesos,abudnik/mesos,jpeach/mesos,reneploetz/mesos,asamerh4/mesos,dforsyth/mesos,abudnik/mesos,gsantovena/mesos,reneploetz/mesos,gsantovena/mesos,kaysoky/mesos,kaysoky/mesos,chhsia0/mesos,Gilbert88/mesos,reneploetz/mesos,chhsia0/mesos,chhsia0/mesos,reneploetz/mesos,kaysoky/mesos,kaysoky/mesos,abudnik/mesos,dforsyth/mesos,dforsyth/mesos,dforsyth/mesos,asamerh4/mesos,asamerh4/mesos,jpeach/mesos