Add special case handling of 0-length netmasks in NFS helper.
This is required because nfs-utils on Linux support CIDR style
network access unless the netmask is length 0.
Also split out utility function for wrapping IPv6 addresses with
square brackets.
Closes-bug: #1707946
Change-Id: Id907478837099b250a6c88b6b5ff50c214bcdbb2
