Add support to use different resource_string functions

Due to some specific needs when accessing resources inside eggs, it was
needed to support other resource_string functions (not just the
environ.get_resource_string one).

It is possible to any SlaveView subclass to override get_resource_string
function to use its own method of extracting resources.

Change-Id: I44e78dce1778a9481603c1bc24607f3435b5f39d