allow couchdb container to mount persisted filesystem path

[rabbah]

in #4245 one of the capabilities that will be lost is having a persisted volume for the db - I propose adding this back to the cleaned couchdb playbook so that it's possible to keep the db bits from one deployment to the next (wipe and init playbooks will work as expected, but in this case removing the files that are persisted and re-initializing them).