Add discogapic generator

[chingor13]

Java Compute uses a discogapic generator (the a gapic generator that accepts discovery docs rather than protos). This adds a new generator that fetches the discovery-artifact-manager repo.

The java compute synth would end up like:

import synthtool as s
import synthtool.gcp as gcp

gapic = gcp.DiscoGAPICGenerator()
common_templates = gcp.CommonTemplates()

library = gapic.java_library(
    service='compute',
    version='v1',
    config_path='artman_compute.yaml',
    artman_output_name='')

s.copy(library / 'gapic-google-cloud-compute-v1/src', 'src')

[chingor13]

I opted for a separate module rather than trying overload the other gapic generator. An alternative would be to add another option to _generate_code() for the repo to use. The downside is that it's another repo to sync in the setup.

[theacodes]

I am a little concerned about the now duplicated code, especially the low-level stuff around invoking docker and figuring out the config file. Can you work with @busunkim96 to factor that out?

[chingor13]

@theacodes @busunkim96 offered to do the refactor and should be able to push directly here.

[chingor13]

Looks like the git import was removed in the discogapic generator but is used to checkout the discovery artifact manager repo. (edit: fixed before I submitted my comment :))