Properly handle base64 enconding of certificates stored as bytes.

This change is needed to properly handle base64 encoding of certificates
stored as bytes, under Python 3, as used by IPA service. It does not
affect Python 2.7 as bytes are identical to str in this version of the
language.

When retireving certificates stored by FreeIPA service data is returned
as bytes, under Python 3, and encoding then breaks, as there is no
bytes.public_bytes method. In Python 3, encoding with base64 will be the
same for strings and bytes.