Split out BaseTypeFactories

Split out BaseTypeFactories

Instantiating BaseTypes with complex definition is a prime example
of where either a builder or a factory pattern is appropriate. Given
we are dealing with a few types with very few customizations, this
patch applies the factory pattern to the problem.

This makes all BaseType subclasses properly immutable and has the
benefit of squashing instances to singletons when they are not
otherwise customized.

Change-Id: I86d09324624c1e5b1a73a0be31269f2233fc15b3
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>