core: modify uri location of engine get session user service

part of ovirt-engine uri rework, to make it more well behaved
application.

the location of the servlet was moved.

/OvirtEngineWeb/ValidateSession ->
/ovirt-engine/services/get-session-user

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=961677
Change-Id: Ie90780b44759afb94cd34f5bf931e9bc0e6f7dba
Signed-off-by: Alon Bar-Lev <alon.barlev@gmail.com>