lxd-images: Add support for image syncing
lxd-images: Add support for image syncing

This does the following:
 - Introduce new --quiet global switch
   When passed, all non-error output will be hidden.

 - Introduce new --sync ubuntu importer switch
   When passed, an image property will be set, this property is what the
   sync command looks for.

 - Introduce new "sync" command
   This command will look for any image which was imported with --sync,
   check if a newer version of the image is available and if so, update the
   image to it.

 - Drops redundant image hashing
   Instead of locally hashing the downloaded files, just forward the
   fingerprint that's available in the simplestream data.

 - Improves pre-existing image detection
   When importing an Ubuntu image that's already in the store, no
   download will occur, instead, any extra alias passed on the command line
   will be set.

Closes #1161

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

dustinkirkland/lxd,jsavikko/lxd,techtonik/lxd,lxc/lxd,stgraber/lxd,Skarlso/lxd,dustinkirkland/lxd,ericsnowcurrently/lxd,akshaykarle/lxd,ericsnowcurrently/lxd,mjeanson/lxd,hallyn/lxd,Skarlso/lxd,Malizor/lxd,chrisglass/lxd,stgraber/lxd,erikmack/lxd,mjeanson/lxd,erikmack/lxd,atxwebs/lxd,lxc/lxd,stgraber/lxd,erikmack/lxd,ollie314/lxd,stgraber/lxd,techtonik/lxd,stgraber/lxd,georghopp/lxd,ollie314/lxd,Malizor/lxd,stgraber/lxd,akshaykarle/lxd,techtonik/lxd,hallyn/lxd,jsavikko/lxd,dustinkirkland/lxd,hallyn/lxd,Skarlso/lxd,yura-pakhuchiy/lxd,lxc/lxd,atxwebs/lxd,ericsnowcurrently/lxd,pcdummy/lxd,hallyn/lxd,akshaykarle/lxd,ollie314/lxd,chrisglass/lxd,lxc/lxd,chrisglass/lxd,mjeanson/lxd,lxc/lxd,pcdummy/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,pcdummy/lxd,hallyn/lxd,Skarlso/lxd,yura-pakhuchiy/lxd,jsavikko/lxd,Malizor/lxd,yura-pakhuchiy/lxd,achanda/lxd,georghopp/lxd,achanda/lxd,georghopp/lxd,achanda/lxd,atxwebs/lxd,atxwebs/lxd,stgraber/lxd