Revamp Mock configuration - non-turing complete config parsing

[praiskup]

Configuration file parsing should be a safe action, so we can drop the
work-around we have (privilege drops, etc.). Per older issue:

exec is a security risk and a code smell whether run as a regular user
or as root; regular users have lots of important data and secrets too.
The comment even acknowledges this...

Yes, long-term we should migrate out from the format of the configuration
files have now (python code with exec()), but that will be a huge change,
and a lot of stuff will need to be changed in all the infrastructure using
Mock.

Originally posted by @praiskup in #990 (comment)