Add support for capturing packets on packet generator interfaces

Add support for capturing packets on packet generator interfaces

This patch introduces following changes:
 - 4 predefined pg/stream[0-3] interfaces are removed
 - Interface naming is changed form pg/streamX to pgX where X can be
   any u32 value
 - one pgX interface can handle multiple streams
 - keyword "source pgX" is added to "packet-generator add" command, X is 0
   by default
 - new cli "packet-generator capture" is introduced
 - new cli "create packet-generator interface pgX"

Change-Id: I768d075b9d4a34f0b5073debdc5dd4a0880c682c
Signed-off-by: Damjan Marion <damarion@cisco.com>