Make ssh's ControlPath configurable via ansible.cfg

This shouldn't generally be needed unless you're working in an environment
that uses rediculously long FQDNs; if the name is too long, you wind up
hitting unix domain socket filepath limits enforced by ssh.

While adding that functionality, adjust the parsing of ansible.cfg so that it stops trying to
interpolation w/in that file itself (ini style configs have %(blah)s style interpolation).