twemproxy: allow /etc/default/twemproxy to be passed in

Allows the instantiation of the twemproxy module to configure its behavior by
passing a file reference or file URI pointing to a file the content of which
should be used to populate /etc/default/twemproxy.

Change-Id: Ia0b13c965b66e54e706cf606969bfea6114c83f2
Merge "twemproxy: allow /etc/default/twemproxy to be passed in" into production