make-release: abstract out version numbers

I wanted to stop passing version and abstract out decomposedVersion
somewhere else.  The new MwVersion is passed the raw version number (ie:
'1.20.1'), run the decomposition and let one easily access the major
version/previous branch whatever.

The various methods relying on versions now use the MwVersion object via
self.version.  That makes the code slightly more readeable.

Change-Id: Ib0ec3fc46e28a285d5af7ee3e584616450b285f9