init.common: Create dhclient file if needed

On redhat/centos, the file didn't existed and lead to an error making
everything exiting.
init: Adding set_path()

On redhat/centos, it's important to force the PATH to insure the shell
to find its binaries.
init.common: Using give_up if no network

give_up is made to handle an error properly regarding user's will. Don't
try to reinvent how to proceed if no network found.
health: Adding ONSUCCESS|ONFAILURE support

As per pxe role, let's support ONSUCCESS and ONFAILURE