Move the assignment of autoinstall_default to the relevant block
where it's being tested.

Provided autoinstall data is already validated against a specific
schema by the relevant controller, but these schemas themselves
are never validated against a meta schema to guarantee they are
valid JSON schemas. This patch adds unit tests for each controller,
as well as SubiquityServer, to validate their autoinstall schema
against the latest JSON meta schema draft (that is supported in
jsonschema). A particular draft validator can be used instead by
specifying meta schema in the schema definition.

SubiquityServer is responsible for checking and loading the
interactive-sessions, so it should be responsible for validating
this section as well. Additionally add this to the autoinstall
schema in the docs.

Adds a base exception type for Autoinstall related failures and a
specific implementation for autoinstall validation failures.

When a user passes incorrect autoinstall data, the installer will crash
with an AutoinstallValidationError exception. Failure messages are
currently in the form of:

    "Malformed autoinstall in '<autoinstall_key>' section"

where <autoinstall_key> is the name of the top-level key a particular
controller is responsible (e.g., 'apt' and MirrorController).

The section reporting is a little crude in the validation
of the base schema done by SubiquityServer, which can't discern
between the 'interative-sessions' and 'version' keys, but for now
the scope is pretty limited and can be fixed up at a later time.

Autoinstall related failures are more likely than not going to be
user caused, so we shouldn't immediately generate a crash report
for these types of failures. This should hopefully allow the user
to debug their autoinstall data much faster and reduce the number
of autoinstall-related bugs reported.