NetTestSlave: add missing import of ExecCmdFail

Method restore_system_config is using exception ExecCmdFail that is not
imported. This results in an exception. The patch adds missing import
statement.

Signed-off-by: Jan Tluka <jtluka@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>