script: better database initialization

• BETTER Uses SQLAlchemy and SQLAlchemy-Utils to initialize the database instead of executing mysql in a python subshell. (closes #2846) (closes #2844)

• NEW Adds support for PostgreSQL database initialization.

Signed-off-by: Leonardo Rossi <leonardo.r@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>