Prune from child gear app-deployments dir

Modify distribute implementation so it keeps the entire app-deployments
dir (excluding the "current" symlink) in sync between the proxy gear
and the child gears. Previously, each new deployment would create a new
deployment dir on the children in app-deployments without pruning older
deployment dirs, eventually filling up a gear's quota.

Bug 1038634
Merge pull request #4290 from ncdc/bz1038634

Merged by openshift-bot