boxcutter_sonatype should detect whether or not on pro or not

[taylorific]

Some operations like updating groups, return errors saying that they are only supported on Pro:

Deploying to groups is a PRO-licensed feature. See https://links.sonatype.com/product-nexus-repository

Should detect whether or not on pro to short-circuit operations like this.

[taylorific]

Seems like the key issue is that should not set 'group_writable_member' on a non-PRO-licensed nexus