Configure FQDN as Hostname for Pods

<!-- This section is incredibly important for producing high quality user-focused documentation such as release notes or a development roadmap. It should be possible to collect this information before implementation begins in order to avoid requiring implementors to split their attention between writing release notes and implementing the feature itself. KEP editors, SIG Docs, and SIG PM should help to ensure that the tone and content of the Summary section is useful for a wide audience. A good summary is probably at least a paragraph in length. --> This proposal gives users the ability to set a pod’s hostname to its Fully Qualified Domain Name (FQDN). A new PodSpec field setHostnameAsFQDN will be introduced. When a user sets this field to true, its Linux kernel hostname field (the nodename field of struct utsname) 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 setHostnameAsFQDN will default to false to preserve current behavior, i.e., setting the hostname field of the kernel to the pod's shortname. Related Kubernetes issue #1791.