lxc-ubuntu-cloud: various small changes

lxc-ubuntu-cloud: various small changes

 * ppc64el images now exist and generally function.
   Instead of failing because an arch isnt in the list,
   let that check happen by ability to download something.
 * update the hard coded ubuntu releases to know about 'trusty'
   and drop no longer supported releases (consistent with behavior
   when distro-info is available)
 * shorten the logic that decides if host and container arch
   are supported.
 * support skipping "invalid arch" check entirely via undocumented
   variable UCTEMPLATE_SKIP_ARCH_CHECK.
 * update usage to reference 'tryreleased' as the default 'stream'
 * give good error message if user tries 'released' and there
   is no released version available.

Signed-off-by: Scott Moser <smoser@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>