Improve proto generation and cleanup old protos

[albscui]

While attempting to move proto generation to pachyderm/pachyderm, we've learned a few things and improved the scripts for this process. However, we decided that proto generation and packaging is a bigger project that involves the organization as a whole, so we won't be migrating proto generation from python-pachyderm to pachyderm/pachyderm atm. Related issue here pachyderm/pachyderm#6116.

However, we can still benefit from this exercise and apply some of the improvements:

• Simplified and more valid proto/run for generating protos
• Parameterize the proto generator's output dir
  • this will allow us to generate different protos based on different versions of pachyderm/pachyderm
• Slimmer Docker image using python:3.6-slim

We are also adding a new python_pachyderm.proto.v2 namespace to store the protos generated from pachyderm v2.x

We removed existing generated protos from python_pachyderm/proto.

Next steps

• We can generate a set of protos based off of pachyderm/pachyderm v1.x
• Update python_pachyderm/__init__.py to defer importing a specific proto version, so that python_pachyderm.proto.v1 and python_pachyderm.proto.v2 can co-exist in the same repo

Relates to #282

[msteffen]

LGTM!