Add bash_completion to swiftclient

This patch basically follows the bash completion
model that other OpenStack clients use. It creates
a new command to swiftclient called `bash_completion`.

The `bash_completion` command by default will print
all base flags and exsiting commands. If you pass
it a command, it'll print out all base flags and
any flags that command accepts. So as you type out
your swift command and auto-complete, only the current
available flags are offered to you.

This is used by the swift.bash_completion script to
allow swift commands to be bash completed.

To make it work, place the swift.bash_completion file
into /etc/bash_completion.d and source it:

  cp tools/swift.bash_completion /etc/bash_completion.d/swift
  source /etc/bash_completion.d/swift

Because swiftclient itself is creating this flag/command output
it should automatically add anything we add to the swiftclient
CLI.

Change-Id: I5609a19018269762b4640403daae5827bb9ad724

Author: matthewoliver