The values for 'network_interface' and 'neutron_external_interface' are
missing from all.yml, meaning it is impossible to override them on a per
node / per group basis. (globals.yml get's top precedence).
Make these consistent with the rest of the variables and move the
defaults into all.yml. Operators can still override / update these in
globals.yml as before, but those wanting more flexibility now have it
via host / group variables.
Change-Id: I2575921f76a8e245106da765757c70353bd6762c
Closes-Bug: #1604129
158a852998