JCLOUDS-306. added public acl to openstack-swift

Note this does work on HPCS, but doesn't work on Cloud Files, as they prohibit anonymous reads.  It remains the case that Cloud Files needs a vendor-specific CDN hack in order to expose a container publicly.

Thread from IRC:
```
notmyname
adrianc: FWIW, x-container-read does work AFAIK for granting access to another CF user. it just doesn't work for public content
notmyname
ie public access non-CDN is specifically detected and prevented
```