Bug 6787: FeatureConfigPusher confusing WARN log removed

see analysis in bug: Current code already has a retry loop, and the
operation eventually succeeds; the ConcurrentModificationException
logged only created confusion and added no real value.

The code has been changed to log the last exception in the ERROR IFF
after N retries it does not succeed.

PS: We should create a generic utility helper for this ("Retryer").

Change-Id: I3c116e77f5a94366da15cc659b3a63d3a6e79f18
Signed-off-by: Michael Vorburger <vorburger@redhat.com>