Reduce number of required environment variables for bot_local.py

Currently we assert the presence of these variables:

    'SLACK_ACCESS_TOKEN',
    'BOT_ACCESS_TOKEN',
    'GITHUB_ACCESS_TOKEN',
    'NPM_TOKEN',
    'SLACK_SECRET',
    'TIMEZONE',
    'PORT',
    'PYPI_USERNAME',
    'PYPI_PASSWORD',
    'PYPITEST_USERNAME',
    'PYPITEST_PASSWORD',

Some environment variables like SLACK_ACCESS_TOKEN are not useful for bot_local.py, so we should only assert them when the Slack bot is starting.

Also, we should consider finer grained verification of environment variables at the command level. For example release does not require PYPI_USERNAME at all, and publish may or may not require PYPI_USERNAME depending on whether it's a python or npm release. I think it's fine to validate all of these variables on boot for Doof but it makes simple commands difficult for bot_local.py

Start in bot_local.py by tracing the boot-time environment-variable assertions and how commands such as release and publish are dispatched. Determine which variables each command and bot startup actually needs. Done means simple bot_local.py commands no longer require unrelated variables while Doof still validates its full set at boot.