Start at least two Glance API workers, limit it to four in total

When only one Glance API worker is running, long running operations
like streaming an image can block the API when it uses a native
backend that is not greenlet'ified (like for example rados) for
several minutes, depending on the image size. Start at least two
API workers to increase chance that the API is still available.
Fixed bug where glance image cache stall timeout set in the glance
add/edit form does not take effect due to incorrect attribute being used.

See also image_cache_stall_timeout in:
        bc-template-glance.json
        _edit_attributes.html.haml
Merge pull request #154 from cdearborn/pull-req-release-roxy-master-a5ab246768b7aacb97a6e161dbcbed0632b0a8c5

Got +1 and got tested => merging.
Merge pull request #150 from dirkmueller/glance_workers

Got +1 and got tested => merging.