Fix ExternalOutput with Simulcast

[jcague]

Description

Fix ExternalOutput with Simulcast. It now gets the lower quality layer, which is sub-optimal. But having a better solution would involve:

• Implement a pipeline in ExternalOutput to detect quality layers and filter packets.
• Connect two ErizoJS (one acting as a pure MCU and another recording the stream).

[] It needs and includes Unit Tests

Changes in Client or Server public APIs

Not needed.

[] It includes documentation for these changes in /doc.

[lodoyun]

LGTM, I think it's a good compromise until ExternalOutput can be refactored