Use all uplink interfaces not only interface 0 at OLT

Not a Bug but no other Issue type is suiting: This is a added feature.

In the current state VOLTHA only allows to use the first uplink NNI at the OLT, only changing the mapping of physical ports to the identifiers (0..N) allows to use another NNI. Still, only one uplink is usable and subscriber flows can't be programmed for a specific uplink NNI.

To enable all uplink NNI at the OLT and deploying the subscriber flows with a specified uplink NNI  through the whole VOLTHA stack from ONOS to the OLT hardware, I propose modifications/additions to the following components (start at OLT hardware):

openolt

• https://gerrit.opencord.org/c/openolt/+/34962/
• https://gerrit.opencord.org/c/openolt/+/35151
• CI probably fails as modification in voltha-protos is required and proto files are pulled from github

voltha-protos

• https://gerrit.opencord.org/c/voltha-protos/+/34960
• https://gerrit.opencord.org/c/voltha-protos/+/34992

openolt-adapter

• https://gerrit.opencord.org/c/voltha-openolt-adapter/+/34993

onos-olt

•  https://gerrit.opencord.org/c/olt/+/35043

The voltha core already passes the messages containing the NNI correctly.