Create new evaluator for each evaluation run

MoM used one evaluator for all the policy runs. That caused weird
side effects because the stack already contained old values from
previous runs.

Policies could not be evaluated properly because we changed defvar
semantic from set to setdefault in one of previous patches.

Change-Id: I3d5343e94d8ecddca0e0c9dd7fc893d94e751378
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1025845
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21704
Reviewed-by: Adam Litke <alitke@redhat.com>