Improvements to the purge-deps.sh script
Improvements to the purge-deps.sh script

- If the log directory does not exist, create it
- The deps or build-deps directory may not exist, since they are
  independent (for example, for centos-queens we do not have build-deps).
  In that case, ignore the non-existing directory for the purge.

Change-Id: I3a4ade8dbc83926f598d47f2236e8efc4fb5beed

javierpena/puppet-delorean,javierpena/puppet-delorean,javierpena/puppet-delorean,rdo-infra/puppet-dlrn,rdo-infra/puppet-dlrn,rdo-infra/puppet-dlrn