Use new string formatting interface

The Python documentation mentions issues with printf-style string formatting and recommends the new str.format() interface [0]. The new interface is used in some parts but not everywhere. The attached patch fixes this by using the new interface where printf-style is currently used.

Additionally, it solves several bugs in lib/chutney/TorNet.py caused by the printf-style separator (%) being outside of the print(). The bugs result in Chutney crashing when tor or tor-gencert is not in PATH and not specified through the environment variables.

[0] https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting