core: scheduler: change parameter map type

The external scheduler is now expecting a string map with vm id instead
of vm entity, therefore, changing the parameter map to hold strings (instead of objects),
and passing the vm entity as a seperate parameter.

Change-Id: Ie0475218e2b66fcd48faf0b92ac2873c4b448c0e
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>