Enable CSV logging output for DStat.

Future work toward visualization of DevStack and devstack-gate performance
would benefit greatly from the availability of machine-parsable DStat output.
This patch outputs an additional logfile to $LOGDIR, `dstat-csv.log`, using
DStat's built-in CSV logging functionality.

An additional instance of DStat is started during start_dstat that outputs
to CSV-formatted text without `--top-cpu-adv` and `-top-io-adv` enabled, as
these plugins are currently incompatible with CSV output. To facilitate this,
a new `dstat.sh` script is added to $TOP_DIR/tools/ to act as a daemon to
manage the two processes.

Change-Id: I826c94c35b6a109308b4f132c181ff7a1f63bc7b
Depends-On: I534fb1f9356a7948d2fec0aecc7f275e47362a11

lib/dstat

@@ -19,8 +19,7 @@ set +o xtrace
 # start_dstat() - Start running processes, including screen
 function start_dstat {
     # A better kind of sysstat, with the top process per time slice
-    DSTAT_OPTS="-tcmndrylpg --top-cpu-adv --top-io-adv"
-    run_process dstat "dstat $DSTAT_OPTS"
+    run_process dstat "$TOP_DIR/tools/dstat.sh $LOGDIR"
 
     # To enable peakmem_tracker add:
     #    enable_service peakmem_tracker