Add of environments variables to allow default paths overriding

* Added OPENFLUID_USERDATA_PATH and OPENFLUID_TEMP_PATH environment
  variables to override default paths if needed
* Updated tests for using these variable,
  and avoid external interactions while testing
* Update OpenFLUID CMake module
* Updated Travis-CI configuration file
* Fixed OpenFLUID market client when using temp directory
* Fixed VectorDataset in OpenFLUID-landr when using temp directory

(closes OpenFLUID/openfluid#372)