CP-2190 for PR-1221: monitor memory_free_kib instead of memory_total_kib

Also fix an Exception string.

Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>
Fixed the description of memory_total_kib rrd datasource.
It is memory "in the host", not "in use".

Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>
Merge pull request #991 from thomassa/pr1221

CP-2190 for PR-1221: fix memory monitoring