Fix tasks for standalone and fix average progress validation error

[johnbaldwin]

https://appsembler.atlassian.net/browse/BLACK-2554

Fix average progress validation error

The problem was that the new pipeline workflow did not round enrollments' "progress percent" when averaging for CourseDailyMetrics average_progress fixed floating point field. This commit addresses this

Fix site handling for standalone mode

This commit makes figures.sites.get_sites aware of standalone mode. The purpose of this is to help make standalone and multisite modes transparent to the bulk of Figures code

There are also a couple of minor changes in figures.tasks

1. populate_daily_metrics_next now includes the exception string to the error log entry
2. run_figures_monthly_metrics is now standalone aware. Since only one site is run for standalone, there's no need to spawn the site level function as a subtask

After this PR, I'll do a PR bump for version to 0.4.3

[johnbaldwin]

@OmarIthawi Thanks for approving!