Consistently use Rake::Task#invoke
Consistently use Rake::Task#invoke

TL;DR: use #invoke when we are trying to call a Rake Task

Wordy: #execute is correct only when:
- the task has no prerequisite or you explicitly want to skip the
  dependencies
- the task takes no args

Signed-off-by: Jesse Zhang <4e375e814be137a4f94f9d2b2a5f4ca930cc7a5a@pivotallabs.com>

barthy1/bosh,barthy1/bosh,barthy1/bosh,barthy1/bosh