add python-mongoengine to deps (el7 only)

This commit adds python-mongoengine for el7 builds. An EL6 version is already
available via EPEL.
FTR this goes against the fedora packaging guidelines, but it's probably not worth modifying this upstream spec file unless we have to. http://fedoraproject.org/wiki/Packaging:Python_Eggs#When_to_Provide_Eggs