Refactor codebase and unit tests to default to execution
under Python 3.
Drop install shim as Python 3 is always present >= trusty.
Drop legacy dhcp and network reassignment code from charm as
a) this relies on a py3 neutronclient (not supported on older
releases) and b) this function was superceeded by the ha-legacy-mode
and then neutron router and network HA built in functionality.
Use charmhelper provided get_host_ip as this superceeds the in
charm version of this function.
Change-Id: I0b28bf0851d44e85b1e856cbd97b71099faa76ae
b63f8687f9