Add ability to configure Dotmesh zpool location

[prisamuel]

User requirement

For the Dell PoC we're looking at using a Dell Isilon store available as an NFS. We'd like to add some configurability to specify a mount point to use for the zpools.

Note from Alaric:
Feature needed: If the storage is on NFS, we'll need some configurability as to putting our zpool on there (probably a good idea anyway). The things we have in place for e2e tests to use /dotmesh-test-pools might be adaptable to that purpose!

Note from Luke:
If you zpool create pool in a location manually before installing everythig, dotmesh will pick that up.

[alaric-dotmesh]

This can be bodged by going to the machines and creating a zpool before installing anything, but it would be a useful feature to be able to do this in the dotscience configuration (especially for runners, perhaps through the GUI when creating the runner).