Add compose.mkdtemp which creates and returns a unique temporary
directory under <compose_topdir>/work/{global,<arch>}/tmp[-<variant>]/
Change tempfile.mkdtemp in code to compose.mkdtemp, so temporary
dirs are created under the compose's workdir, this makes it more
easier to maintain the temporary dirs/files especially when compose
fails with amount of temporary files left there.
Signed-off-by: Qixiang Wan <qwan@redhat.com>