Allow the Nuxeo random importer to generate a non uniform tree

By default when calling the /randomImporter endpoint of the Bulk document importer package to perform a mass import of a randomly generated tree, the number of children per node and the Folder vs File distribution is not really random.

Since 8.3 the nonUniform query parameter allows to have a non uniform distribution of the number of children per folder with approximatively:

• 10% folders with a of big number of files (~= 5000)
• 10% folders with 1 file
• 80% folders with a random variation of the default number of files, so between 50 and 150

This is interesting to perform more realistic load tests.
It was required to validate the batched implementation of the Nuxeo Drive remote scan (NXP-19388 and NXP-19482) and check the difference with the recursive implementation (NXP-19209).