This is for *StandaloneDeploymentScriptTest* test suites, which
download a large disk image during slapos node instance step.

For example, on a Wendelin.StandaloneDeploymentScriptTest-Debian.Stretch
test run [1], we can see that the slapos node instance started at
2022-05-13 07:50:30,788 and stopped trying at 2022-05-13 07:51:35,885.

In the last run, slapos node instance failed, especially this promise:

    2022-05-13 07:51:35,701 INFO     slapgrid_cp: 2022-05-13 07:51:35 slapos[3548917] ERROR ERROR '/srv/slapgrid/slappart22/t/cjl/i/1/srv/monitor/private/virtual-hard-drive-url/virtual-hard-drive-url-processed-config.state' not empty, content available at https://[???]:8026/private/virtual-hard-drive-url/virtual-hard-drive-url-processed-config.state
    ...

and if we access this URL, we can see the error:

    [Errno 2] No such file or directory: '/srv/slapgrid/slappart22/t/cjl/i/1/var/virtual-hard-drive-url/update-image-processed.md5sum'

which means that the image is still not downloaded.

testnode still ignore slapos node instance state return code and run
tests anyway, so the test ran and failed later.

This image is 1.8Go, when I tried to download it locally it took
56.41 seconds, so it must be generally almost OK.

By retrying up to 60 times, we give more time for the slapos node
instance step to download the image.

[1]: https://softinst161166.host.vifib.net/cjl-oSgtg7v20D/suite.log