Backend.ai-krunner-alpine

* Embed the licenses of CPython and pre-installed wheels and make them available via
`/opt/backend.ai/licenses` in user containers.

* Replace the ttyd build with a custom built one with lower libwebsockets version
to mitigate frequent connection timeout issue with ttyd service ports.

* Update ttyd version to 1.6.1.

* Update dependencies and match the plugin interface.

* Update the krunner image for `alpine3.8` to Python 3.8.2 along with krunner dependencies.

* Add `ttyd` as an intrinsic service binary.