set_virtual_boot_device is also using the import system configuration
action, but its usage is very specific, with retries and rebooting. To
keep it simple, this adds import_system_configuration, which is
asynchronous and returns a TaskMonitor.
Additionally, this changes the case of the header field names used by
the asynchronous.http_call method to make them work with unit tests.
While the requests package can handle real header field names case-
insensitively, when they are mocked in unit tests, case needs to match.