sentry: always override init settings with the environment variables

Instead of only overriding the initial settings when they're left unset,
always override the defaults when the environment variables are set.

This makes the behavior more consistent with the way we usually handle
environment variables. It also allows setting the environment variable
SWH_SENTRY_DISABLE_LOGGING_EVENTS=false, to enable events based on the
logging framework in gunicorn backends (where the default has been
flipped to true).