Add gbs zsh completion support #693

Add gbs zsh completion support #693

~ $ gbs
build        -- local build package
changelog    -- update the changelog file with the git commit messages
chroot       -- chroot to build root
clone        -- clone a git repository
export       -- export files and prepare for build
import       -- import spec file/source rpm/tar ball to git repository
pull         -- update a package git repository
remotebuild  -- remote build package
submit       -- submit tag to gerrit and trigger building in OBS

~ $ gbs build -A
armv6l   armv7hl  armv7l   i586     x86_64

~ $ gbs changelog -
--conf                    -c  -- specify config file for gbs
--debug                   -d  -- debug output
--help                    -h  -- show this help message and exit
--message                 -m  -- use given message as the changelog entry
--packaging-dirdirectory      -- containing packaging files
--since                   -s  -- commit to start from
--spec                        -- specify a spec file to use. It should be a file name that GBS will find it in packaging dir
--verbose                 -v  -- verbose output
--version                 -V  -- show program's version number and exit

Change-Id: I049a3ad30cb6aed385e6c6421c594f7de421e5b8
Signed-off-by: LingchaoX Xin <lingchaox.xin@intel.com>