Resync charm helpers to pickup fixes to support veth wiring of
OVS bridges directly to Linux bridges created with MAAS.
Linux has a maximum interface name length of 15 charms; the
change ensures that the generated interface length for the veth
pair fits within this limitation.
Previous behaviour (that worked) is preserved to avoid duplicate
veth wiring or direct wiring of the Linux bridge to OVS.
Change-Id: I294dc7c05b7c08503ef200707e0f9b1d1c199843
Closes-Bug: 1773353
2a7749333c