Retries for shrinking root disk make testing less flakey
Retries for shrinking root disk make testing less flakey

[#157010541](https://www.pivotaltracker.com/story/show/157010541)

Signed-off-by: Jason Smith <a9a653d4151fa2c081ba1ffc2c2726f3b80b7d7d@pivotal.io>

mattcui/bosh-agent,gu-bin/bosh-agent,mattcui/bosh-agent,gu-bin/bosh-agent,cloudfoundry/bosh-agent,mattcui/bosh-agent,cloudfoundry/bosh-agent,gu-bin/bosh-agent