[3210] Changes after review:

 - tests/methods descriptions improved
 - CfgMgrTest.echoClientId improved
 - Redundant checks removed from Dhcpv4SrvTest.discoverEchoClientId
 - BIND10 Guide clarified
 - ChangeLog added
[3210] Redundant statements in Dhcpv4SrvTest.requestEchoClientId removed
[3210] Changes after review:

 - flag => configuration parameter
 - echoClientId test uses ASSERTs rather than EXPECTs