gp: Convert CA certificates to base64

gp: Convert CA certificates to base64

I don't know whether this applies universally, but in our case the
contents of `es['cACertificate'][0]` are binary, so cleanly converting
to a string fails with the following:

'utf-8' codec can't decode byte 0x82 in position 1: invalid start byte

We found a fix to be encoding the certificate to base64 when
constructing the CA list.

Section 4.4.5.2 of MS-CAESO also suggests that the content of
`cACertificate` is binary (OCTET string).

Signed-off-by: Gabriel Nagy <gabriel.nagy@canonical.com>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: David Mulder <dmulder@samba.org>