Change Pools to be able to clear healthMonitor property

This patch changes healthMonitor property of Pool unsettable.

This fixes MN-2375.

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Add the description about type of Health Monitor

This patch adds the description about type property of Health Monitor in
the CLI doc.

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Merge pull request #185 from tfukushima/mn2375-v1.4

Change Pools to be able to clear healthMonitor property