add a random cdmi-enterprise-number in the config
fix cdmi tests

- add variable to activate cdmi in test config
- fix the RestResource class to apply the path prefixes
    given on the backend interface (e.g. /tmp/new/ or /zone/user/)
    correctly
- lower the constraints for the cdmi object ID
refactor http_storage code

- put stream_generator() in http_storage.common
  (used in cdmi and noncdmi)
- use the iter() function to loop over file contents
- refactor _get_cdmi_filters: split into two functions
 other small things
speed up objectID calculation

use ''.join for string concat instead of s1 + s2
allow any value for the objectID of the root ('/') parent

the cdmi spec does not say anything if the parent of the root dir
should have an objectID, but it suggests that all entries, called
by name, so also the root entry, should have a parent path and parentID.
in the case that the root dir is not '/', there might be even some sense
in displaying e.g. /srv/ for the root: /srv/01/ to construct the full
path from both.
this patch relaxes the tests so that we don't check the parentID of the
root dir, because there is no clear way of handling it.
enable the cdmi tests