Add local override

[agraubert]

This allows files to be downloaded by worker nodes outside of the NFS.
Added two new localizer arguments:

• localizer.local_download_dir: Where canine temporary disks will be downloaded. {download_dir}/{disk_name}/. Defaults to /mnt/canine-local-downloads/{random string}
• localizer.temporary_disk_type: Either 'standard' or 'ssd'. Controls type of disk that gets provisioned. If set to None, then disks will not be provisioned. Downloads are saved to the local download dir without mounting a new disk there

Closes #55

[coveralls]

Coverage decreased (-0.1%) to 59.58% when pulling bf0b26b on local_download into f1c1cd9 on master.