Update dCacheView to use a configurable tag when selecting WebDAV doors

[paulmillar]

Currently, dCacheView uses the tag dcache-view when selecting which WebDAV door(s) to use for data transfers.

If there are multiple frontend/dCacheView for different customers then it may be desirable to select a different set of WebDAV doors for different frontend instances.

One way to achieve this is to allow the selection tag to be configurable.

This is one possible solution to #250.