Fix create_export/remove_export in driver.py

Fix create_export/remove_export in driver.py

1. There was a call to rpcapi.create_export which does not have a
matching volume manager function, causing volume migration to crash.
This call was not necessary because there is already an
initialize_connection RPC call, which calls the driver's create_export
function. Removed the create_export RPC call and function. Added better
error handling to that code in _attach_volume in driver.py as well.

2. The manager called remove_export from detach_volume, which was not
being called by these functions. I believe it makes more sense to call
it from terminate_connection. Moved it there, and fixed up the
corresponding code in _detach_volume in driver.py.

3. Remove leftovers of export code from create_volume manager flow.

Change-Id: I2b192630ebed54368f151db47b49cbc72601a8d7
Closes-Bug: #1285060