Add option to export/import encrypted geli keys when saving/restoring config using legacy UI

After fresh installing FreeNAS and uploading saved config, attempting to unlock an encrypted pool results in the GUI returning "Error Unlocking: [object Object]". Exporting the pool then attempting to reimport it results in the folling error:

" EFAULT] The following devices failed to attach: gptid/3bd6360c-33f7-11e9-b986-08626649c154, gptid/3a6e95bc-33f7-11e9-b986-08626649c154, gptid/3948e17e-33f7-11e9-b986-08626649c154, gptid/3834894c-33f7-11e9-b986-08626649c154, gptid/371776ef-33f7-11e9-b986-08626649c154, gptid/35feaee0-33f7-11e9-b986-08626649c154"

The traceback for this is:

Error: Traceback (most recent call last):
File "/usr/local/lib/python3.6/site-packages/middlewared/job.py", line 332, in run
await self.future
File "/usr/local/lib/python3.6/site-packages/middlewared/job.py", line 365, in __run_body
rv = await self.middleware.run_in_thread(self.method, *([self] + args))
File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 1005, in run_in_thread
return await self.loop.run_in_executor(executor, functools.partial(method, *args, **kwargs))
File "/usr/local/lib/python3.6/concurrent/futures/thread.py", line 56, in run
result = self.fn(*self.args, **self.kwargs)
File "/usr/local/lib/python3.6/site-packages/middlewared/schema.py", line 668, in nf
return f(*args, **kwargs)
File "/usr/local/lib/python3.6/site-packages/middlewared/plugins/disk.py", line 253, in decrypt
raise CallError(f'The following devices failed to attach: {", ".join(failed)}')
middlewared.service_exception.CallError: [EFAULT] The following devices failed to attach: gptid/3bd6360c-33f7-11e9-b986-08626649c154, gptid/3a6e95bc-33f7-11e9-b986-08626649c154, gptid/3948e17e-33f7-11e9-b986-08626649c154, gptid/3834894c-33f7-11e9-b986-08626649c154, gptid/371776ef-33f7-11e9-b986-08626649c154, gptid/35feaee0-33f7-11e9-b986-08626649c154