Use with to factor out try/excepts

Also, add 'ignored' contextmanager

Signed-off-by: Andy Grover <agrover@redhat.com>
Have discovery_enable_auth return bool

Signed-off-by: Andy Grover <agrover@redhat.com>
Preliminary support for NetworkPortal iser property

Get/set, dump() and setup() support.

Untested.

Signed-off-by: Andy Grover <agrover@redhat.com>