Title: #853: x509,certdb: handle certificates as bytes
This will require some more refactoring, adding WIP label.
Basically, `Bytes` parameter only base64-decodes its value if it's of `unicode`/`str`
instance (Python2/Python3), but in most cases, its value is checked with
`validate_ceritifate()` of the `ipaserver.plugins.service` module, which requires the
certificate to be in DER format. But the ceritificate won't be decoded to DER from PEM
as it above decoding condition is not fulfilled.
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