Due to eventlet bug in GreenPool implementation when first
spawned thread threw exception imap method exited immediately
without waiting for other spawned threads to accomplish.
Issue in eventlet github:
https://github.com/eventlet/eventlet/issues/232
This commits wraps executed functions so that no exception can be
raised on imap boundary and then unwraps result after execution
Closes-Bug: #1449497
Change-Id: I0e99de8e0711071c4a009e37a58ec22761e90e6c