Refactor vars_plugins (host/group_vars)

Split out parsing of vars files to per host and per group
parsing, instead of reparsing all groups for each host. This enhances
performance.

Extend vars_plugins' API with two new methods:
* get host variables: only parses host_vars
* get group variables: only parses group_vars for specific group
The initial run method is still used for backward compatibility.

Parse all vars_plugins at inventory initialisation, instead of
per host when touched first by runner. Here we can also loop through
all groups once easily, then parse them.
This also centralizes all parsing in the inventory constructor.

	modified:   bin/ansible
	modified:   bin/ansible-playbook
	modified:   lib/ansible/inventory/__init__.py
	modified:   lib/ansible/inventory/vars_plugins/group_vars.py
Introduce noop vars plugin.

In preparation to move the group_vars plugin code into core.

	new file:   lib/ansible/inventory/vars_plugins/noop.py
Move inventory.set_playbook_basedir from ansible-playbook to playbook constructor
Move group/host_vars parsing into core inventory

	modified:   lib/ansible/inventory/__init__.py
	deleted:    lib/ansible/inventory/vars_plugins/group_vars.py
	modified:   lib/ansible/utils/__init__.py