Adds user-selectable hash merging support in vars

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.
Fixes hash merging

No need for deep copy (vars are essentially immutable once loaded, there
should not be any modifications)
Merge pull request #1903 from leucos/mergeable-hash-vars

Adds user-selectable hash merging support in vars