osc build on remote --host

The new argument for osc build --host will perform the build on a remote
host. It is a shortcut for

rsync -az -e ssh `pwd` user@hostname:/remote/dir
rsync -az -e ssh prefer-dir \
user@hotname:/remote/dir/__prefer-rpms__/prefer-dir
ssh -t user@hostname "cd /remote/dir/package; osc build *build-args"
rsync -az -e ssh user@hostname:/remote/dir/__keep-pkgs__ keep-pkgs

iow it copy the current directory to the /remote/directory on hostname
(if not specified, the ~/ is supplied) and then run the osc build on
hostname. All global and local arguments are supplied to the remote osc
build, but arguments points to local path (--host, --keep-pkgs, --prefer-pkgs,
--rsync-dest, --rsync-src and --overlay) are modified to match to remote
host.

It support the OSC_ variables, so running with
OSC_BUILD_ROOT=/somewhere osc build --host ... will push the variable to
the remove osc process
- documented "osc build --host..." feature in the NEWS file