Modify empty parameter handling in RCSDiscoveryManager

Modify empty parameter handling in RCSDiscoveryManager
and tranfer "RCSDiscoveryTask"'s location into "RCSDiscoveryMananger".

- if input value(relativeUri) is empty, will be discovered after be changed into "OC_RSRVD_WELL_KNOWN_URI".
- delete white space.
- According to move "RCSDiscoveryTask" into "RCSDiscoveryManager", changed name from "RCSDiscoveryTask" to "DiscoverTask".

Change-Id: I544d001e5b3a2f0fda75452f86430aceee2b46e5
Signed-off-by: doil.kwon <doil.kwon@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/2663
Reviewed-by: Madan Lanka <lanka.madan@samsung.com>
Tested-by: Madan Lanka <lanka.madan@samsung.com>