tools: Improve engine-config help

This patch provides a new option to engine-config -

engine-config -h <keyname>

For example - engine-config -h AdminPassword

This will provide more specific help (comparing with engine-config -h)
to keys.

Change-Id: I090d52cbc246eb9ffdd985ca6b5aeab6582c8653
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>