vdsm: capture warnings in log only if supported

python_warnings_enable is off by default but let's protect ourselves
if we forget that logging.captureWarnings is not in Python 2.6

Change-Id: Iefd185defc17431a76f4c943c381b71e9477426e
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34509
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>