openshift-install should configure User-Agent

openshift-install makes many calls to OpenStack APIs when installing OpenShift on OpenStack. Currently all of these calls use the same default User-Agent header gophercloud/x.y.z, where x.y.z is the version of the gophercloud that openshift-install was built with.

Keystone logs the User-Agent string, as do other OpenStack services, and it can provide important information about who is interacting with the cloud. As recently seen in OCPBUGS-14049, it can also be useful when debugging issues with components.

We should configure the User-Agent header for openshift-install and all other OpenShift components that talk to OpenStack APIs.