Bundle Yamale schema

TIL! Using the existence of obscure member sys._MEIPASS, I can detect if
we're running from a bundled PyInstaller binary, versus running from Python
directly.

Add schema.yaml to the datas of the PyInstaller spec. Then, if the
-/--schema flag is given, use it, and if it's not given, default to the
built-in one if we're running from a bundled binary, or fall-through to
./schema.yaml in other cases.

This avoids the need for config/schema.py as a carbon-copy of the schema,
slick!

schema.yaml

@@ -1,6 +1,3 @@
-### NOTE(pim): This file is the source of truth for the Yamale schema validator.
-###            Make sure to copy this file into config/schema.py's yamale_schema
-###            when it is changed here.
 interfaces: map(include('interface'),key=str(),required=False)
 bondethernets: map(include('bondethernet'),key=str(matches='BondEthernet[0-9]+'),required=False)
 loopbacks: map(include('loopback'),key=str(matches='loop[0-9]+'),required=False)