Add API to return host stats history

GET /host/stats/history will return the host stats history - the last 60
values.
That way the grid in UI can show the history when displaying the host
tab and then use /host/stats to get the new updated values - item by
item.

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
Host stats history: Update test cases

Add new tests to verify the URI /host/stats/history

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
Display historical host statistical info on the host tab

Update the host tab statistics graphs to make use of the newly available historical data.
Removed option to continue collecting data while the host tab is out of view, as the server
always has the historcal data available.

Signed-off-by: Adam King <rak@linux.vnet.ibm.com>