Add Traffic Ops dynamic volume number calculation

[rob05c]

Currently, Traffic Ops volume.config (and storage.config volume number) is created from parameters, Disk_Volume, RAM_Volume, and SSD_Volume.

There's no reason to calculate volume numbers from user input. They can be calculated from the Drive_Prefix parameters. For example, Drive_Prefix only gets volume 1. If Drive_Prefix and SSD_Drive_Prefix exist, Disks get volume 1 and SSDs get volume 2.

This should be a relatively small and safe change. The logic that needs changed/added is in traffic_ops/app/lib/UI/ConfigFiles.pm sub volume_dot_config and storage_dot_config (and a comment in hosting_dot_config).

Putting this in the 1.8 milestone because it isn't critically important, and it can wait for 2.0 if 2.0 is out soonish. But if it's a long time to get out, we should go ahead and do this in 1.x, because it's not big and makes one less thing to go wrong in production.