Clever Runners: `cc_max_flavor` config item

It can be interesting for some runners to opt out of the biggest flavors, either to avoid overloading the infrastructure or just because it decreases the size of launchable jobs that the runner can get from the coordinator.

For a practical example, when developing the Clever Runner, I often use a testing configuration with a small quota (especially if working on the quota system itself). But setting a quota smaller than the weight of the 3XL flavor results in no poll occurring, since the quota is smaller than the minimum size that can be requested (no flavor-aware requesting is currently possible, hence that actually means the biggest available).

We'd probably set it to 2XL the runner of foss.heptapod.net#196 (closed), reserving thus 3XL to the commercial service.

Also the biggest flavor used in the Heptapod group is XL, hence it could benefit from declaring the self-restraint.