importlib not python 2.6 compatible

[bari12]

Motivation

For the dynamic lfn2pfn function the module importlib is used. However, importlib is not part of python 2.6, thus it breaks the clients used on the ATLAS local root base. One option would be to use the backport of importlib for python 2.6 (on pypi) - needs to be investigated.

https://pypi.python.org/pypi/importlib

• Adding importlib to the dependencies works for python 2.6 and 2.7, but a install on python 3.x fails like this.

For now we move the import to a conditional part of the code, as no module gets dynamically loaded at the moment anyway; This needs to be addressed later on with the dynamic loading of permissions and policies, as this should be handled in a similar way.