Avoid "Due to a method name conflict in roles..." errors

Avoid "Due to a method name conflict in roles..." errors

When composing into a role that chooses to define a
_get_default_configfile method, we would conflict with the existing
method already defined here.  Simply not defining one, and checking
if one exists with ->can, is nicer than forcing users to say
{ -excludes => '_get_default_configfile' }.