Use namespace-aware meta importer for ipaplatform

Instead of symlinks and build-time configuration the ipaplatform module
is now able to auto-detect platforms on import time. The meta importer
uses the platform 'ID' from /etc/os-releases. It falls back to 'ID_LIKE'
on platforms like CentOS, which has ID=centos and ID_LIKE="rhel fedora".

The meta importer is able to handle namespace packages and the
ipaplatform package has been turned into a namespace package in order to
support external platform specifications.

https://fedorahosted.org/freeipa/ticket/6474

Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>

ipapython/setup.py

@@ -42,6 +42,7 @@ if __name__ == '__main__':
             "dnspython",
             "gssapi",
             # "ipalib",  # circular dependency
+            "ipaplatform",
             "netaddr",
             "netifaces",
             "six",