Fix user guide typos and text

  Updated ubuntu user to format with <literal> in
  /openstack-manuals/doc/user-guide/section_dashboard_launch_instances.xml,
  and changed "username" to "user name."

  Updated "etc" to "and so on," changed "Heat" to "Orchestration,"
  and changed intro paragraph about Orchestration to match what's on the
  OpenStack getting started site in:
  /openstack-manuals/doc/user-guide/section_dashboard_stacks.xml

Change-Id: Ie33d9a31c9d8b1ada31bf1c226ccb92eb0c08f1f
Merge "Fix user guide typos and text"