[2.x] Failure to render drivers.yaml apt key in curtin_userdata

[Mike Pontillo]

When we go to render the a preseed containing a custom driver, we have the following code in the template:

    {{py: key_string = ''.join(['\\x%x' % x for x in map(ord, driver['key_binary'])])}}

As of Python 3, driver['key_binary'] is a binary string, not a text string, and so ord() causes a crash. What we really need is:

    {{py: key_string = ''.join(['\\x%x' % x for x in driver['key_binary']])}}

Workaround: edit /etc/maas/preseeds/curtin_userdata and modify the aforementioned line of code.