standalone: strip ansi code in logs

This fixes a bug when the program output contain some ansi code, like the \e[0;31m ansi code to display text in color on the terminal. These characters are not allowed in XML so if they end up in the XML-RPC messages this cause the underlying expat library to complain that the stream is not well-formed (invalid token).

Fortunately, supervisor supports this and has a strip_ansi option that we can set in the config to prevent these problems.