ZTP: Enhancements to support both DHCP options and PHC

You can use either the legacy DHCP-options-based zero-touch provisioning (ZTP) or the phone-home client (PHC) to provision software for your device. If the device boots and receives DHCP options from the DHCP server for ZTP, ZTP resumes. If DHCP options are not present, PHC is attempted. PHC enables the device to securely obtain bootstrapping data, such as a configuration or software image, with no user intervention other than having to physically connect the device to the network. When the device first boots, PHC connects to the preconfigured Juniper redirect server (redirect.juniper.net), which will redirect to a phone-home server (PHS) to get the configuration or software image. To initiate either DHCP-options-based ZTP or PHC, the device must either be in a factory-default state, or you can issue the request system zeroize command.