Create app environemnt variable bookshelf_url_ttl in app config and
replaced GET_URL_TTL with DEFAULT_BOOKSHELF_URL_TTL.  Added url_ttl
function to use variable if it's defined, otherwise use the default
value.
For consistency use S3 in TTL config value naming.

* DEFAULT_BOOKSHELF_TTL => DEFAULT_S3_TTL
* bookshelf_url_ttl => s3_url_ttl
Merge branch 'move_url_ttl'