[bugfix] [shared/transforms/cpe_generate.py] Fix bug when generating cpe-oval file:

When searching for cpe_variables to be included into final cpe-oval file don't
traverse whole ovaltree for 'var_ref's, but just tree under the 'tests' leaf.
Since otherwise this leads to incorrect non cpe_variables inclusion of local_variables
into final cpe-oval file, leading to datastream build failure.

See e.g: https://lists.fedorahosted.org/pipermail/scap-security-guide/2014-August/006025.html
for further details.