Add '--clear-all-labels' and '--clear-no-labels' to 'con repos init

[zanerock]

When we init a repo, it loads the Conveyor standard issues labels and clears out the default github labels. In some cases, the user may want to clear all their pre-existing labels.

In other cases, the user may want to keep the git labels. In both instances, we're thinking primarily of maintaining the labels until they can be migrated.