Execute glance upload in a native thread as it may block the current
coroutine until it completes.
Despite the fact we use eventlet monkey_patching [1] to achieve cooperative
yielding for network IO, file IO on busy file system may still get
nova-compute hanging.
Stick those IO in a native thread using eventlet tpool.execute() [2]
avoid this issue.
[1] https://eventlet.net/doc/patching.html
[2] https://eventlet.net/doc/threading.html
Closes-Bug: #1874032
Change-Id: I8dbc579e0037969aab4f2bb500fccfbde4190726
Alexandre Arents