ovpurgejournals should purge journal files for all regions

[Derek_]

Currently, ovpurgejournals removes the oldest default.mjl_* journal files to keep no more than a maximum number of journal files as specified in journals.conf. The functionality needs to be generalized to allow for other regions.

Similar to Bug #818551 for ovswitchjournals, ovpurgejournals should have an option for specifying the region:

ovswitchjournals -r DEFAULT dev

But it also needs to allow for num_journals, specified in journals.conf, to be specified differently for each region, especially since different regions will grow their journals at very different rates. Alternatively, the control of journals might be changed to be based on dates or some other constraint that could work uniformly for all regions.