* Refactoring of "packages" Python package
* Now MuranoPL YAML classes can be parsed by different engines
depending on Format string in package manifest
With this change MuranoPL classes will no longer
be read and parsed (including YAQL expressions parsing)
upon package load but only on first class access and
with ability to peak different parsers depending on
format version specified in manifest. As a consequence
it is now possible to use different YAQL engines for
different package format versions. Also startup
time was greatly improved as unneeded classes are no more
parsed and logos and UI forms are not loaded.
Partially implements: blueprint murano-versioning
Change-Id: I23fc5da1a43b405d526438329dc04aed589dee13
9a9f3436e1