Reduce number of required environment variables for bot_local.py

[noisecapella]

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