allow pep 621 configuration

allow pep 621 configuration

Added optional :pep:`621` support to Alembic, where a subset of the
project-centric configuration normally found in the ``alembic.ini`` file
can now be retrieved from the project-wide ``pyproject.toml`` file.   A new
init template ``pyproject`` is added which illustrates a basic :pep:`621`
setup.  The :pep:`621` feature supports configuration values that are
relevant to code locations and code production only; it does not
accommodate database connectivity, configuration, or logging configuration.
These latter configurational elements remain as elements that can be
present either in the ``alembic.ini`` file, or retrieved elsewhere within
the ``env.py`` file.   The change also allows the ``alembic.ini`` file to
be completely optional if the ``pyproject.toml`` file contains a base
alembic configuration section.

Fixes: #1082
Change-Id: I7c6d1e68eb570bc5273e3500a17b342dfbd69d1b