Post-install script for easier setup & upgrades

Let's add a post-install script to our manager which performs:

- Run `alembic upgrade head` or `backend.ai mgr schema oneshot` (if first install)
- Run any version-specific migration scripts (named with a fixed rule) if present in specific package location (e.g., `migrations` directory)\* The migration directory must be included as package data so that it is distributed as part of wheels.

This must be executed after all of the following commands:

- `pip install -U backend.ai-manager`
- `pip install -U -e .` in the working copy
- `python setup.py develop`
Currently there is no simple way to add pre/post-install scripts in Python wheels.(Adding codes around `setup()` or setting `cmdclass` does not seem to work for all cases above...)We need to figure out how to do this first.

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JIRA Issue: BA-308