Limit the number of jobs per campaign, per user, per site.

[jgaida]

Limiting the number of jobs per user, per site is especially useful for staying within the chart or to avoid submission errors due to admission rules (On G5K, an admission rule limits the number of jobs per user, per site).

[bzizou]

Currently, there's the "max_jobs" JDL directive; but it is a per campaign/per cluster value.
Then, we should add the possibility for an administrator to configure per user/per cluster max number of jobs value. I think that we need to implement a new table for that. But before, we should clarify the idea of "site" that is not currently a known thing into the Cigri code. See also #18