Add some more checks and constraints

[naved001]

After #185 is merged we should add some additional checks.

• [ ] See a list of all provisioned nodes and what disks they are provisioned from. This one will require us to track the states in a database. This is required for the next item in this list.
• [ ] Refuse to delete an image if a node is provisioned from it.

cc: @ianballou @apoorvemohan