Add 'sandbox' configuration key and build-uid/build-gid elements

This only affects SandboxBWrap at the moment.

buildstream/_loader.py: Add Symbol.SANDBOX and allow it in validation
buildstream/_metaelement.py: Add 'sandbox' variable and store it in the object
buildstream/_project.py: Add 'sandbox' configuration key and load it from
    project.conf.
buildstream/data/projectconfig.yaml: Default build-uid/build-gid values of 0
    for 'sandbox'.
buildstream/element.py: Add __extract_sandbox_config to find the final sandbox
    configuration. Pass this to the sandbox constructor.
buildstream/sandbox/_sandboxbwrap.py: If sandbox configuration was supplied,
    use it for uid and gid instead of the default 0.
buildstream/sandbox/_sandboxchroot.py: Throw exception if non-0 uid/gid were
    supplied.
buildstream/sandbox/__init__.py: Import SandboxConfig.
buildstream/sandbox/_private.py: New file, containing SandboxConfig. Made private
to avoid documentation for this class.