Backend.ai-manager

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- Technical release to fix backend.ai-common depedency version.

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- Allow separate upgrade of the manager from v1.2 to v1.3 by extrapolating a new
"kernel_host" field in the return value of the internal krenel creation RPC call.

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- Now the Backend.AI gateway uses a modular architecture where you can add 3rd-party
extensions as aiohttp.web.Application and middlewares via `BACKEND_EXTENSIONS`
environment variable. (65)
- Adopt aiojobs as the main coroutine task scheduler. (65)
Using this, improve handler/task cancellation as well.
- Public non-authorized APIs become accessible without "Date" HTTP header set. (65)
- Upgrade aiohttp to v3.0 release. (64)
- Improve dockerization support. (62)
- Fix "X-Method-Override" support that was interfering with RFC-7807-style error
reporting. Also return correct HTTP status code when failed route resolution.

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- 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)

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- Minor update to fix dependency versions.

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NOTICE
- From this release, the manager and agent versions will go together, which indicates
the compatibility of them, even when either one has relatively little improvements.
CHANGES
- The gateway server now consider per-agent image availability when scheduling a new
kernel. (29)
- The execute API now returns exitCode value of underlying subprocesses in the batch
mode. (60)
- The gateway server is now fully horizontally-scalable.
There is no states shared via multiprocessing shared memory and all such states are
now managed by a separate Redis instance.
- Improve logging: it now provides multiprocess-safe file-based rotating logs. (10)
- Fix the Admin API error when filtering agents by their status due to a missing
method parameter in `Agent.batch_load()`.