The install path of knife is changed (install check failing)

At plugins/provisioners/chef/cap/linux/chef_installed.rb,
It supposed the path of knife to 'opt/chef/bin/knife'.
But the real path is /usr/bin/knife on version 11.8.2

```
vagrant@vagrant-ubuntu-trusty-64:/etc$ which knife
/usr/bin/knife
vagrant@vagrant-ubuntu-trusty-64:/etc$ knife --version
Chef: 11.8.2
```

So the check method chef_installed should be changed.

I don't make testcase for this because I don't know how to make a testcase for this case.
I hope someone help it.

Thank you for reading this request.