utilities: Add bash command-line completion script.

This patch adds bash command-line completion script for ovs-appctl,
ovs-dpctl, ovs-ofctl and ovsdb-tool command.  Right now, the script
can do the following:

- display available completion or complete on unfinished user input
  (long option, subcommand, and argument).

- once the subcommand (e.g. ofproto/trace) has been given, the
  script will print the subcommand format.

- the script can convert between keywords like 'bridge/port/interface/dp'
  and the available record in ovsdb.

The limitations are:

- only support small set of important keywords
  (dp, datapath, bridge, switch, port, interface, iface).

- does not support parsing of nested options
  (e.g. ovsdb-tool create [db [schema]]).

- does not support expansion on repeatitive argument
  (e.g. ovs-dpctl show [dp...]).

- only support matching on long options, and only in the format
  (--option [arg], i.e. should not use --option=[arg]).

To use the script, either copy it inside /etc/bash_completion.d/
or manually run it via . ovs-command-compgen.bash.

Also, a unit testsuite is provided as ovs-command-compgen-test.bash.
It is suggested that this test script be run only inside
tutorial/sandbox directory.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>

---
V2 -> V3:
- refine the script and test framework.
- make sure the completion script works fine when there is no ovs
  started or installed.
- add necessary documentations.

RFC -> V2:
- Make the extract_prompt() function more reliable based on Peter's
  suggestion.
- Extend the script to support more commands.