ceph-rest-api: make main program be "shell" around WSGI guts

Move WSGI guts to pybind/ceph_rest_api.py for importability
into WSGI server frameworks.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
ceph-rest-api: separate into module and front-end for WSGI deploy

To deploy ceph-rest-api within a WSGI server (apache/mod_wsgi,
nginx/uwsgi, etc.), there needs to be an importable (.py) module
that performs all init/config when imported.  ceph-rest-api was
close, but it needs to be named properly, and there's no argument
passing, so it needs to get args from a fixed file or the env.

Separate most of ceph-rest-api into pybind/ceph_rest_api.py, and make
its arguments come from the environment, and init errors be
ImportError exceptions.  Recase ceph-rest-api as a thin layer that
does the usual setup and arg parsing, and then sets args into the
environment and imports ceph_rest_api.py, catching exceptions and
reporting errors.  This allows standalone execution as usual.
ceph-rest-api grabs a few module globals (addr/port and the flask.app)
to use after it imports.

Accept cluster name, and do the ceph.conf search using cluster name
in the appropriate places in the searched-for files.

Also ceph_rest_api.py gets a little cleanup (fewer global variables,
cleaner conf file search algorithm, better error reporting on conf
load)

Also: doc updates, packaging updates to include ceph_rest_api.py

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge branch 'wip-wsgi' into next

* wip-wsgi:
  ceph-rest-api: separate into module and front-end for WSGI deploy
  ceph-rest-api: make main program be "shell" around WSGI guts

  Reviewed-by: Sage Weil <sage@inktank.com>