Retry http requests to generate plugins list

This commit retries http requests to generate a plugins list. We often
see internal server errors(500) in the gate. It makes sanity jobs fail.
So, it would be better to retry internally instead of 'recheck'. To
accomplish it, this uses urllib3 module.
NOTE: The default total number of retries[0] is 10. It should be enough
for us.

[0] https://urllib3.readthedocs.io/en/latest/reference/urllib3.util.html#urllib3.util.retry.Retry

Change-Id: Id3da3e2895fc2324d5a2db91f82b12ec5bb610f8
1 file changed