Support for overriding --mesos-root under upstart.

Previously only the sysv init script supported overriding `--mesos-root`
via an env var, and thus via `/etc/default/thermos`.

Bugs closed: AURORA-1647

Reviewed at https://reviews.apache.org/r/45179/
diff --git a/specs/debian/aurora-executor.thermos.upstart b/specs/debian/aurora-executor.thermos.upstart
index 7ebe4be..78b11b7 100644
--- a/specs/debian/aurora-executor.thermos.upstart
+++ b/specs/debian/aurora-executor.thermos.upstart
@@ -22,6 +22,7 @@
   [ -r /etc/default/thermos ] && . /etc/default/thermos
   exec start-stop-daemon --start --exec /usr/sbin/thermos_observer -- \
     --port=${OBSERVER_PORT:-1338} \
+    --mesos-root=${MESOS_ROOT:-/var/lib/mesos} \
     --log_to_disk=NONE \
     --log_to_stderr=google:INFO
 end script