Support dynamic lookup of instance types -> disk mount locations

[prisamuel]

Changing hub_instance_type = "t3.medium" to a t2 type device gives this error on startup

2020-04-23 11:43:41.220951 I | Mounting volume
2020-04-23 11:43:41.220992 I | Running command mount "$HUB_DEVICE" /opt/dotscience-aws || (set -euo pipefail && mkfs -t ext4 "$HUB_DEVICE" && mount "$HUB_DEVICE" /opt/dotscience-aws) with env [HUB_DEVICE=/dev/nvme1n1]
mount: /opt/dotscience-aws: special device /dev/nvme1n1 does not exist.
mke2fs 1.44.1 (24-Mar-2018)
The file /dev/nvme1n1 does not exist and no size was specified.
2020-04-23 11:43:41.238777 I | Command mount "$HUB_DEVICE" /opt/dotscience-aws || (set -euo pipefail && mkfs -t ext4 "$HUB_DEVICE" && mount "$HUB_DEVICE" /opt/dotscience-aws) failed with exit status 1
panic: exit status 1