Customizing groups_vars and host_vars directory structure

As i wanted to create a working directory that would fit my needs, i coded this small patch that allows 3 new vars in the `ansible.cfg` file, named `vars_path`, `group_vars_name` and `host_vars_name`.

I setup those regarding the current logic in `constants.py` with a default values so nothing would break if not customized. The actual customization i've been doing in my `ansible.cfg` file is : 
```
[defaults]
log_path          = ./logs/ansible.log

hostfile          = ./inventories/hosts
roles_path        = ./roles

vars_path         = ./vars
group_vars_name   = group
host_vars_name    = host
```
so my working directory would look like : 
```
.
├── README.md
├── ansible.cfg
├── inventories
│   └── hosts
├── logs
│   └── ansible.log
├── playbooks
│   └── default.yml
├── requirements.yml
├── roles
└── vars
    ├── group
    └── host
```

Mostly, as i think about playbooks as objects, this enhancement would be also effective for playbooks as simple folders having a similar *but lighter* directory structure as roles :

```
└── playbooks
    └── myplaybook
        ├── main.yml
        └── vars
            ├── group
            └── host
```