Memoize get_prj_pseudometa results

[aplanas]

To avoid hit multiple times the OBS server asking for the _meta content in the staging project, I use a memoization version of get_prj_pseudometa.

This will cause problems with the select command, that will update the description field with some YAML metadata, causing get_prj_pseudometa returning outdated information.

To resolve this issue I also provide a mechanism to externally invalidate the memoization content from one specific set of parameter, adding a new dynamic method to the class. So for every memoized method, a new _invalidate_XXXX method is created.