helm: post-upgrade hook to run alembic upgrade

[mvidalgarcia]

* Shall we start thinking about adding a "Upgrade" section to the docs, explaining admins that they should  run ` kubectl exec deployment/reana-server reana-db alembic upgrade`?  Or shall we even add it to post-helm deployment instructions for good?  _(can be done also later)_

We can create a post-upgrade hook to execute reana-db alembic upgrade.

Originally posted by @mvidalgarcia in reanahub/reana-db#128 (comment)

In addition, we can introduce a new brief section in the wiki about how to use reana-db alembic commands.