Fqdn hostname KEP

This proposal gives users the ability to set a pod’s hostname to
its Fully Qualified Domain Name (FQDN). A new PodSpec field `fqdnInHostname`
will be introduced. When a user sets this field to true, its Linux kernel
hostname field ([the nodename field of struct utsname]http://man7.org/linux/man-pages/man2/uname.2.html))
will be set to its fully qualified domain name (FQDN). Hence, both uname -n
and hostname --fqdn will return the pod’s FQDN. The new PodSpec field `fqdnInHostname`
will default to `false` to preserve current behavior, i.e., setting the hostname
field of the kernel to the pod's shortname.

Related issue is: kubernetes#1797