Rename 'debops.interfaces' to 'debops.ifupdown'

The new name should more closely reflect what the role is supposed to
configure. And with the possibility to add a NetworkManager or
systemd-networkd role in the future it becomes possible to have
different roles to manage network interfaces without shared variable
namespace.
Rename ifupdown_{external,internal} variables

'ifupdown_external' variable is renamed to
'ifupdown_external_interface'.

'ifupdown_internal' variable is renamed to
'ifupdown_internal_interface'.
Rename ifupdown_auto_ifupdown variable

'ifupdown_auto_ifupdown' variable is renamed to
'ifupdown_reconfigure'.
Rename ifupdown_ifupdown_delay variable

'ifupdown_ifupdown_delay' variable is renamed to
'ifupdown_reconfigure_delay'.
Rename ifupdown_list variable

'ifupdown_list' variable is renamed to 'ifupdown_interfaces'.
Rename ifupdown_default_list variable

'ifupdown_default_list' variable is renamed to
'ifupdown_default_interfaces'.
Rename ifupdown_dependent_list variable

'ifupdown_dependent_list' variable is renamed to
'ifupdown_dependent_interfaces'.
Clean up task syntax in 'debops.ifupdown' role
Update CHANGELOG.md
Merge pull request #168 from drybjed/interfaces_rename

'interfaces' role is renamed, and some code is cleaned up