atf_python: allow test scripts to pass jail options

Test scripts based on atf_python can now pass jail command options via the
'opts' key in the 'vnetX' key of TOPOLOGY.

Reviewed by: melifaro
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D52761