tc-testing: Add command timeout feature to tdc

Using an attribute set in the tdc_config.py file, limit the
amount of time tdc will wait for an executed command to
complete and prevent the script from hanging entirely.

This timeout will be applied to all executed commands.

Signed-off-by: Lucas Bates <lucasb@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>