add bash completion script

This patch add a script rally.bash_completion which enables
auto-completion of rally CLI in bash environmnet when running:

python setup.py install
or
python setup.py develop

test:
rally [tab]
rally ta[tab]
rally task [tab]
rally task de[tab]

to-do:
adding a cache to bash-completion result could speed up auto-completion

Change-Id: Ia4c68ccf3faea73da87a93ce64b36545d9d8411c

setup.py

@@ -27,4 +27,5 @@ except ImportError:
 
 setuptools.setup(
     setup_requires=['pbr'],
+    data_files=[('/etc/bash_completion.d', ['tools/rally.bash_completion'])],
     pbr=True)

tools/rally.bash_completion

@@ -0,0 +1,18 @@
+_rally_opts="" # lazy init
+_rally_flags="" # lazy init
+_rally_opts_exp="" # lazy init
+_rally()
+{
+	local cur prev rbc cflags
+	COMPREPLY=()
+	cur="${COMP_WORDS[COMP_CWORD]}"
+	prev="${COMP_WORDS[COMP_CWORD-1]}"
+    base_opts="`rally bash-completion`"
+    if [ $prev == "rally" ] && [ $COMP_CWORD == "1" ] ; then
+		COMPREPLY=($(compgen -W "${base_opts}" -- ${cur}))
+    else
+		COMPREPLY=($(compgen -W "`rally bash-completion $prev `" -- ${cur}))
+    fi
+	return 0
+}
+complete -F _rally rally