bootstrap: use alternate method for verify certificate

M2Crypto at python-2.6 issues deprecation warning for current method of
acquiring certificate chain. Use an alternate method.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=901470
Change-Id: I3bf1555888f072347f2f2918fc63be3787856083
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/11204
Reviewed-by: Dan Kenigsberg <danken@redhat.com>