Add support for a "site" concept (group of clusters)

[bzizou]

Can we consider that a site is a group of clusters that may be refered as a cluster into user interactions as the JDL?
We then simply have to add a field "site" to the clusters table and handle that in the parts of the code that need it (especially in the jdl library, for example to fix #18 ). It could be also used to implement a per user/ per site limit of the number of jobs (#23)

[bzizou]

Not so easy: the site concept of grid5000 also implies that a site is composed of clusters using the same OAR server that should not be queried multiple times for each cluster (one query per site to prevent from flooding the OAR database)...