Create a bazel build element

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/529)
In GitLab by [[Gitlab user @edbaunton]](https://gitlab.com/edbaunton) on Jul 30, 2018, 16:20

## Background

Create an element that provides support for building with `bazel`.

Specifically, a bst would declare itself of kind `bazel` and BuildStream would take care of generating the `WORKSPACE` file by recursively scanning the dependencies of the element being built.

## Task description

* `bazel` would be executed inside the sandbox (not using a host tool)
* The workspace file would identify the built sources to consume with `new_local_repository` rules; if it encounters a dependency of type `bazel` it would stage the sources rather than the binaries and consume the `BUILD` file to leverage `bazel`.
* `bazel` would need access to a persistent disk cache (use `_set_mount_source`?) between BuildStream runs so as to take advantage of its built in caching. The `bazel` server will actually be torn down between runs but it is said to serialize its state between runs so the startup time will no be prohibitive.
* Toolchain generation: we need to generate correct toolchain for the target platform and have that be considered as part of the key for the element.