Bug 2175 - Migrate frm, statistics and inventory manager to config subsystem

* migrated statistics manager to config subsystem
* updated tests
* patch resolves Bug 1646 and Bug 2079
* Patch Set 6 - add log.warn msg for a fail loading xml ConfigSubsystem input value + fix MIN_REQUEST_NET_MONITOR_INTERVAL_DEFAULT to 3sec (follow Anil's comment)

Change-Id: I67d398c82c3dafbb2ef759d07a80d17ad6656c1e
Signed-off-by: Jan Hajnar <jhajnar@cisco.com>
Signed-off-by: Vaclav Demcak <vdemcak@cisco.com>