Let module params to have default values taken from another module param

This makes it possible to gradually introduce some new variables to a module
without breaking it for older clients.

Good example is file stat "get_md5" (older) and "get_checksum" (newer)
variables, with this patch it is possible to set "get_checksum" to have
default value of "get_md5".

It allows to introduce new variables more gradually without breaking runtime
characteristics for existing playbooks when running using newer version of ansible
(if somebody disalbed get_md5 then obviously they don't want to do get_checksum too)