ipaplatform: Move paths from installers to paths module

Part of: https://fedorahosted.org/freeipa/ticket/4052
ipaplatform: Drop the base authconfig class

As authconfig is a distro-specific tool there is no incentive for
implying that other platforms should implement any authconfig
implementation of their own.

Part of: https://fedorahosted.org/freeipa/ticket/4052
ipaplatform: Fix build warnings

The newly created ipaplatform subdirectories base and fedora were
mentioned multiple times in the specfile, which produced build
warnings.

Part of: https://fedorahosted.org/freeipa/ticket/4052
ipaplatform: Fix misspelled path constant