Fix `command not found` problem at :shell provision
Fix `command not found` problem at :shell provision

I'm not sure why c.vm.provision must be in under the `define` root.
But moving from public method to under `define` fixes the problem.

And I added rm -rf /var/cache/omnibut/{build,pkg} to :shell provision
for building the packge at each 'vagrant up'.

iantalarico/google-fluentd,treasure-data/omnibus-td-agent,GoogleCloudPlatform/google-fluentd,iantalarico/google-fluentd,GoogleCloudPlatform/google-fluentd,iantalarico/google-fluentd,treasure-data/omnibus-td-agent,GoogleCloudPlatform/google-fluentd,treasure-data/omnibus-td-agent