xenapi: Add support for different image upload drivers

This change refactors image upload code for xenapi and adds the
xenapi_image_upload_handler config option.

Preparation for bp nova-direct-image-upload
DocImpact

Change-Id: Icd4e7ef135beddca9ddcc4a880c09e5a1702a93c
Fix eventlet/mysql db pooling code

Eventlet versions >= 0.10 return a tuple from
db_pool.ConnectionPool().create(), not a scalar.

Also, set the FOUND_ROWS client flag, which sqlalchemy
appears to require to sanity check the number of rows
touch.

bug 1097992

Change-Id: I64e798660c506fcd13947a232cb7832db0318360
Merge "Fix eventlet/mysql db pooling code"
Merge "xenapi: Add support for different image upload drivers"