templates: persist appliance metadata on install

Appliance fields that describe the appliance (vendor information, default
credentials, installation instructions...) were dropped when installing a
template. Keep them in a new appliance_metadata JSON column on the
templates table, filled by the appliance-to-template conversion for both
registry v1-6 and v8 (version level values override the appliance level
ones). The nested schema allows extra fields so future registry fields
persist without a migration.

gns3server/db/models/templates.py

@@ -36,6 +36,7 @@ class Template(BaseTable):
     builtin = Column(Boolean, default=False)
     usage = Column(String)
     netmiko_device_type = Column(String)
+    appliance_metadata = Column(JSON)
     template_type = Column(String)
     tags = Column(JSON)
     compute_id = Column(String)