Convert scap-rebuild-cdbs to python

Convert scap-rebuild-cdbs to python and replace mergeCdbFileUpdates with
scap.tasks.merge_cdb_updates which can be called directly from the
python codebase. The new scap-rebuild-cdbs script ensures that is it
caled as the mwdeploy user. This removes the need for an extra layer of
indirection.

Change-Id: Ie9679019ba02edaf55adc8eb100614b6710bf629