engine: external scheduler integration

This patch adds a new ExternalSchedulingManager class to handle
communication with the external scheduler proxy. Other changes:
- added configuration values (scheduler proxy url, enable/disable)
- added external scheduler invocation to vds filtering
- error messages

Change-Id: I95b6660beb319df0afe569f31c16033bf6192df9
Signed-off-by: Laszlo Hornyak <lhornyak@redhat.com>