Openseadragon module: replace version with configurable variable

[bseeger]

The OpenSeadragon (OSD) module code has the version of the OSD to use hard-coded in a few places. It's tricky to make it a configurable variable, but we should do that at some point.

Example: https://github.com/Islandora/openseadragon/blob/8.x-1.x/openseadragon.module#L150

[whikloj]

Forgive my ignorance, I've been out of Islandora 8 for a bit. It appears that there is the OSD config already loaded in the method you linked to. Could you add a field there and pull it when you make that array? Or is the tricky bit the validation of the version?