[r=wallyworld],[bug=1305780] Look for mongo* executables when not in PATH
Backup is performed by a bash script that, among
other steps, dumps the mongodb, this makes use
of mongodump and mongoexport binaries which are
provided by the package juju-mongodb and placed
in a path not included in PATH.
The backup works on some versions of juju that use
mongodb-server instead, I added a check to make
sure that, if present, we use the binaries
provided by juju-mongodb
https://codereview.appspot.com/90660043/