Add tftp datasource for cloud config

[niusmallnan]

This allows for a tftp datasource for cloud-config. This is useful if you have an environment already setup for PXE booting and do not want to serve cloud-config via eg. HTTP.

Usage:

kernel ${base-url}/vmlinuz rancher.cloud_init.datasources=[tftp:1.2.3.4:69/cloud_config]

Port is optional and defaults to port 69.

[niusmallnan]

@kingsd041 We need an automated test for this function.

[kingsd041]

Ros 152-rc1 already supports TFTP as a datasource, I tested ipxe boot and install to disk, it works fine