Set separate interfaces list var using dependency

During playbook run, 'interfaces_list' variable contains list of network
interfaces to configure by 'interfaces' role. This list can be defined
using 'set_fact' module at runtime.

Unfortunately, if a separate play is run after main common play which
adds new interfaces, 'interfaces_list' variable couldn't be overwritten
automatically. This patch changes the role to expect additional,
separate 'interfaces_dependent_list' variable - if it's set (via
dependency variables), it will override all other variables of this type
and allow to define a custom list of network interfaces by another role.

You can still define 'interfaces_list' using inventory variables.
Merge pull request #65 from drybjed/role/interfaces

Set separate interfaces list var using dependency