Post-install script for easier setup & upgrades

[achimnol]

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.

┆Issue is synchronized with this Asana task by Unito

JIRA Issue: BA-308

[achimnol]

Or, we could execute the post-install script whenever the manager server or the CLI starts.
(For the CLI, we could ask the user to confirm the upgrade operation. If the server is running already, we also should warn the users to restart it.)