Fix swift-dispersion-populate bug when using SimpleClient class

Commit 7fcbbebb changed the internal client a little bit and removed the
class variable self.attempts in SimpleClient. This is currently still
needed in swift-dispersion-populate to track the number of retries, thus
re-adding it again.

Closes-Bug: 1351323
Change-Id: I98ba6441607158b34708b2dbb2d1a63788681d63