Add venv environment to tox

Some of the stackforge jobs use an environment "venv" to run
commands, like packaging a release.

Change-Id: I3453aaa0c7201d780301447936d26aca8ecfb91f
Allow a on_load_failure_callback to be provided

When entrypoints are loaded it is sometimes useful
to be able to do more than LOG the failure that could
be emitted from the plugin failing to be constructed,
allowing the manager classes to be provided a callback
that can be called when such extension fails to load
is useful to track these types of failures.

Change-Id: Idc7e55ade6b3f80c348123fbceea64425d7d3508
fix typo in contrib docs

Change-Id: Iae9aa84f48092b1c0d19d7271f049a9ff786996f
Merge "Add venv environment to tox"
Remove requirements checking for dependencies

Change-Id: I4128994554a9291d5629e20774b1c5a91c4a58cc
Merge "Allow a on_load_failure_callback to be provided"