cmd/juju: don't use JobManageNetworking with servers that don't support it

This fixes LP #1402826.
Merge pull request #1322 from mjs/1402826-jobmanagenetworking-compat-1.21

cmd/juju: don't use JobManageNetworking with servers that don't support it

This fixes LP #1402826.

(Review request: http://reviews.vapour.ws/r/641/)