Set default and check existance of Policy Type

Set default and check existance of Policy Type

The TOSCA specification has a "bug" in that it does not have a field to specify
the version of a policy type to use. We already had introduced the "type_version" field
for this.

This review introduces setting of the default version of a policy type to be
be used by a policy as the latest version of the policy type in the database.
As a side effect of this, we now have to check for existence of the policy type
of a policy in the database. This means that creation/update of a policy with
a non-existant policy type specified will now fail.

Issue-ID: POLICY-1738
Change-Id: I27080cf6cd358948810dab6897c72dfe4d41fe91
Signed-off-by: liamfallon <liam.fallon@est.tech>