[v6r13] Native FTS3

This PR includes all the changes necessary to make a proper use of FTS3:

* it splits the configuration between FTS2 and FTS3 (see https://github.com/DIRACGrid/DIRAC/wiki/DIRAC-v6r12)
* The code logic is split, but still shares many points
* A new object FTSPlacement hides the internals of FTS (strategy, graphs...) to all the clients, agents, services
* FTSPlacement creates either an FTS2Placement or an FTS3Placement depending on the configuration
* FTS2Placement is the FTSStrategy and FTSGraph of before
* FTS3Placement is dummy and relies on the FTS3 server
* the submit and monitor of FTS3 jobs is now done with the native python interface (still waiting for an official release)

We still need that this ticket is taken care of #1932 (If not done, we will submit to non healthy FTS servers)

The changes in the CS are mandatory, EVEN if we a VO decides to still use FTS2

This will need heavy testing...

Note: this PR is a rebase on v6r13 of a branch that I had developed and tested against v6r11, before Philippe makes all the staging fixes for FTS2.