Setting the client UserAgent makes it easier to identify vCenter sessions
used by the vSphere Cloud Provider.  This is useful to remove sessions that
have leaked, such as when a VCP process goes away without calling Logout().
And to test that VCP properly re-authenticates when a session is removed.

Example use:
  govc session.ls | grep kubernetes-cloudprovider | awk '{print $1}' | xargs -n1 govc session.rm