FR: Parameterized task labels.

[aiuto]

TL;DR: Allow variable substitution in task name so that something like this works:

common: &common
  name: "{bazel}_{platform}"

lts: &lts
  bazel: latest

rolling: &rolling
  bazel: rolling 

ubuntu: &ubuntu
  platform: ubuntu1804
  <<: *common

windows: &windows
  platform: windows
  <<: *common

tasks:
  rolling_ubuntu:
    <<: *ubuntu
    <<: *rolling
  rolling_windows:
    <<: *windows
    <<: *rolling
  lts_ubuntu:
    <<: *ubuntu
    <<: *lts
  lts_windows:
    <<: *windows
    <<: *lts

The task names should be precisely the names defined by the string, without adding the platform display label or the imported .yml file name.

Motivation:

• When trying to define CI runs for both LTS and rolling, you want the bazel version in the name to disambiguate tasks easily. Example: https://github.com/bazelbuild/rules_pkg/pull/430
• Most rule authors rarely care about the precise JDK running on each platform (the way the Bazel core project does). They care more about the bazel version and OS. Thus, that should not appended to the task name unless desired.

Difficulty: Minimal. A patch that mostly does it is:

$ diff bazelci.py ../continuous-integration/buildkite
921,930c921
<     new_name = namespace
<     if old_name:
<         if old_name.find('{') >= 0:  # }
<             new_name = old_name.format(
<                 namespace = namespace,
<                 **task_config
<             )
<         else:
<             new_name = "%s (%s)" % (namespace, old_name)
<     task_config["name"] = new_name
---
>     task_config["name"] = "%s (%s)" % (namespace, old_name) if old_name else namespace

I can send a complete PR if we like the idea.