Include task-specific shell commands and build flags

**Description of the problem / feature request:**

For example: https://github.com/bazelbuild/bazel/blob/master/.bazelci/postsubmit.yml#L119
```
ubuntu1804_javabase9:
platform: ubuntu1804_nojava
name: ":java: OpenJDK 9 Javabase"
shell_commands:
- sed -i.bak -e 's/^# android_sdk_repository/android_sdk_repository/' -e 's/^#
android_ndk_repository/android_ndk_repository/' WORKSPACE
- rm -f WORKSPACE.bak
build_flags:
- "--javabase=@openjdk9_linux_archive//:runtime"
...
ubuntu1804_javabase10:
platform: ubuntu1804_nojava
name: ":java: OpenJDK 10 Javabase"
shell_commands:
- sed -i.bak -e 's/^# android_sdk_repository/android_sdk_repository/' -e 's/^#
android_ndk_repository/android_ndk_repository/' WORKSPACE
- rm -f WORKSPACE.bak
build_flags:
- "--javabase=@openjdk10_linux_archive//:runtime"
```

The same `ubuntu1804_nojava` platform can have many tasks based on it, each with a different set of `shell_commands` and `build_flags`.

It's useful to perform benchmarking on each of these different environments.