Backend-ai

------------------

- Add support for BACKEND_VFOLDER_MOUNTS environment variable. (21)
This allows use of auto-mounted vfolders when using 3rd-party integrations such as
Jupyter notebook. The format is a list of comma-separated strings for the vfolder
names.

- Individual API Function objects such as Kernel can now have individual
APIConfig objects via optional "config" parameters to static/class methods
and the per-instance config attribute. (20)

- Improve vfolder CLI command outputs.

- Improve scripting support: all CLI commands that fail now return exit code 1
explicitly.

------------------

- Add metadata/aliases for TensorFlow v1.5 kernel images to the default sample configs.

- Polish CI and test suites.

- Add etcd put/get/del manager CLI commands to get rid of the necessity of an extra
etcdcli binary during installation. (lablup/backend.ai15)

------------------

- BackendError is now normal Exception, not BaseException.
This was a mistake in the early stage of development.

------------------

- Fixed vfolder upload API to work with aiohttp v3.

- "vfolder upload" command can now upload multiple files.

-------------------

- Change StreamPty methods to become coroutines to match with aiohttp v3
API changes.