[bug] Configuration reported as errored: false positives

[nemesifier]

Whenever the internet connection is unstable, the default test can fail and the configuration can be flagged as status "error", even though the issues are not being caused by the configuration itself but by the internet connection.

I think a possible solution may be to increase the number of default test retries, but before trying to change this we should test the different options and once a better default value is found we can change it.

I'd increase the default test retries to 10 (we need to update the README): https://github.com/openwisp/openwisp-config/blob/master/openwisp-config/files/openwisp.agent#L167.