Create new thread to collect Kimchi statistics

That way Kimchi statistics is collected independent of server resquests.

Use cherrypy.process.plugins.BackgroundTask to create the new thread to avoid
problems while stopping the server.

Also reorganize how the data is exposed in order to avoid a lot of stats
variables as more statistics will be added to kimchi.

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
Display network I/O rate instead of memory usage

As we can not provide a meaningful memory usage value to the user without an
agent, replace it to network I/O

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
Display disk I/O rate instead of storage usage

We can not provide a meaningful storage usage value to the user without an
agent so replace it to disk I/O

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
KPI: Update API documentation

Add information about how the statistics data are structured to the API.md file

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
KPI: Updates model and mockmodel tests

Add tests to verify if the statistics data are correct structured and if it has
all the required information

Signed-off-by: Aline Manera <alinefm@br.ibm.com>