Isolate backend loading

Pecan uses thread local storage and so if we combine pecan and eventlet
we need to make sure that eventlet is monkey patched before pecan is
first loaded.

To keep the paste.ini file untouched we need to keep the app creation in
service.py but we can't load that from keystone-all because it has pecan
in it.

So we create a new file that can be safely imported before pecan is
loaded.

Change-Id: If7abf1db9859d66c06f7f223056c106292f256fa
blueprint: keystone-pecan
Merge "Isolate backend loading"