Adds user-selectable hash merging support in vars

Since Michael is open to a patch that doesn't change Ansible current behaviour, here is my attempt.

Hash variables are currently overriden if they are redefined. This
doesn't let the user refine hash entries or overriding selected keys,
which can, for some, be a desirable feature.
This patch let the user force hash merging by setting the
hash_behaviour value to "merge" (without the quotes) in ansible.cfg

However, by default, ansible behaves like it always did and if any value
besides "merge" is used ("replace" is suggested in the example ansible.cfg
file), it will also behave as always.

As always, triple check my python...

Thanks for your time.