software/repman: retry requests to prevent flaky test

Test was sometimes failing with:

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FAIL: test_http_get (test.TestRepman)
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Traceback (most recent call last):
  File "slapos-repository/software/repman/test/test.py", line 81, in test_http_get
    self.assertTrue(cluster['isProvision'])
AssertionError: False is not true

It can take a little time before everything is ready and we don't have a
promise for this, because it's really application level.