Add simple caching engine to speed up repeat inventory lookups.

Use the box_name attribute as the host name reference.

Set ansible_ssh_user, ansible_ssh_private_key_file,
and ansible_ssh_host so ansible can access the VM
instances via the generic vagrant credentials.

First run to populate cache

```
%) time ./inventory/vagrant/vagrant.py --list                                                                                                                                                                                                           
{"vagrant": ["cobbler", "gocd", "gocd-agent", "mirror", "sa"]}
./inventory/vagrant/vagrant.py --list  7.24s user 1.37s system 90% cpu 9.497 total
```
Subsequent runs after caching has been populated,

```
%) time ./inventory/vagrant/vagrant.py --list                                                                                                                                                                                                           
{"vagrant": ["cobbler", "gocd", "gocd-agent", "mirror", "sa"]}
./inventory/vagrant/vagrant.py --list  0.05s user 0.03s system 90% cpu 0.080 total
```