Fix ipa-server-install answer cache
Current Answer Cache storing mechanism is not ideal for storing
non-trivial Python types like arrays, custom classes, etc.
RawConfigParser just translates values to string, which
are not correctly decoded when the Answer Cache is parsed and
restored in the installer.
This patch replaces RawConfigParser with Python's standard pickle
module, which is a recommended way for serialization in Python.
https://fedorahosted.org/freeipa/ticket/2054