Eliminate EC2 plugin execution for non-EC2 providers (Vagrant)

Did some benchmarking of ohai on a vagrant VM stood up using ec-metal:
```
[2014-10-01T17:08:02+00:00] INFO: #<Ohai::NamedPlugin::EC2:0x00000001a280e0>:
[2014-10-01T17:08:02+00:00] INFO:     0.000000   0.000000   0.000000 (127.210415)
```
The EC2 plugin was being run because a hints file was present for ec2 (assumedly as a workaround previously). This is 2 minutes added to the ohai portion of every chef-solo run within ec-metal, which means that every "*-ctl reconfigure" command was taking an extra 2 minutes on vagrant.

"time ohai" with and without the hints file removed (respectively):
```
real	2m8.735s
real	0m1.533s
```
I think this change will only write out the ec2.json file on your local machine,  since it's using the `file` resource and not the `machine_file` resource from metal.