Set smoke/gate attributes for tests in "image"

Change-Id: I4bae858dfc5fba2c45e869a58e89fe7c14c3b30b
Implements: blueprint set-gate-attribute
Add a sleep with back-off to retries

The current implementation retries on socket error
without a sleep, and on auth error with a fixed
interval sleep. Replacing all with an exp backoff.
Default sleep time set to 1.5, default backoff 1.01.

Fixes bug #1180213
Change-Id: I764f2aa4f2c0f8e04718d69b842a574db7aca936
Merge "Set smoke/gate attributes for tests in "image""
Merge "Add a sleep with back-off to retries"