HotFix Bug 1581 - DroptestRPC doesn't work with Cbench

* DroptestRpcSender is initialized for command on only, but check
	command has to be case insesitive in DropTestCommandProvider (fixed)
* add input validation

Change-Id: Ia7131442d22b436290058082ed8a374a90ea35f8
Signed-off-by: Vaclav Demcak <vdemcak@cisco.com>