Fix: use yaml to parse netlab environment variables

Using 'eval' to parse environment variables correctly converted values
into ints/bools, but it was hard to pass a list as an environment
variable (you would have to use all sorts of quoting tricks).

Treating environment variable values as YAML strings still results in
str/int/bools, but also allows you to use simpler (YAML) format to
specify list or even dict values.

Author: ipspace

netsim/utils/read.py

@@ -216,7 +216,7 @@ def include_environment_defaults(topology: Box) -> None:
   for k in os.environ:
     v = os.environ[k]                             # Get the variable value
     try:
-      v = eval(v,{'__builtins__': {}})            # Try to evaluate it as bool/int/whatever
+      v = yaml.load(v,Loader=yaml.SafeLoader)     # Try to evaluate it as bool/int/whatever
     except:
       pass                                        # Can't do that? No harm, we'll assume it's a string