FQDN_MAX_LEN has been defined at multiple locations
in neutron tree, I encountered a situation where use
of this constant from neutron/extensions/dns.py ended
up in cyclic import. The better solution would be to
have it in neutron lib and use it everywhere.
Related-Bug: #1653780
Change-Id: Ifd13f6d85bd7c7f5f803eaa8aac0e16c0f0edff7