Polish Xen bootstrap
Align OTP bootstrap to the Xen one
Provision both Xen and OTP - for LING
Optimize OTP download
Show log in case of build error, and hide irrelevant output
Script download and sample invocation of LING build

Name LING provisioner so that I can call:
```shell
vagrant provision --provision-with ling
```
wip optimize otp download