sysutils/iocage: Update CONFLICTS specification

This port used to restrict its CONFLICTS entry to py-iocage built
for Python-2.7, but it does obviously conflict with that ports built
for any Python version.

This port is marked as NO_ARCH=yes and I do assumed that there is
only an install conflict, but have not tested that assumption and
therefore not changed the CONFLICTS to CONFLICTS_INSTALL.

Approved by: grembo (maintainer)