awscli: Install less to fix help docs

Without less, we get:

```
2016-01-22 10:31:59,098 - MainThread - awscli.help - DEBUG - Running command: ['groff', '-man', '-T', 'ascii']
2016-01-22 10:31:59,109 - MainThread - awscli.help - DEBUG - Running command: ['less', '-R']
2016-01-22 10:31:59,111 - MainThread - awscli.clidriver - DEBUG - Exception caught in main()
Traceback (most recent call last):
File "/nix/store/jmgflkyf1yfz6an82pvhq084mp1j0bjn-awscli-1.7.41/lib/python2.7/site-packages/awscli/clidriver.py", line 183, in main
return command_table[parsed_args.command](remaining, parsed_args)
File "/nix/store/jmgflkyf1yfz6an82pvhq084mp1j0bjn-awscli-1.7.41/lib/python2.7/site-packages/awscli/help.py", line 245, in __call__
self.renderer.render(self.doc.getvalue())
File "/nix/store/jmgflkyf1yfz6an82pvhq084mp1j0bjn-awscli-1.7.41/lib/python2.7/site-packages/awscli/help.py", line 81, in render
self._send_output_to_pager(converted_content)
File "/nix/store/jmgflkyf1yfz6an82pvhq084mp1j0bjn-awscli-1.7.41/lib/python2.7/site-packages/awscli/help.py", line 86, in _send_output_to_pager
p = self._popen(cmdline, stdin=PIPE)
File "/nix/store/jmgflkyf1yfz6an82pvhq084mp1j0bjn-awscli-1.7.41/lib/python2.7/site-packages/awscli/help.py", line 90, in _popen
return Popen(*args, **kwargs)
File "/nix/store/6a1x9r9wywa27v9gvp0yqgq0mmkfa3ww-python-2.7.11/lib/python2.7/subprocess.py", line 710, in __init__
errread, errwrite)
File "/nix/store/6a1x9r9wywa27v9gvp0yqgq0mmkfa3ww-python-2.7.11/lib/python2.7/subprocess.py", line 1335, in _execute_child
raise child_exception
OSError: [Errno 2] No such file or directory
2016-01-22 10:31:59,112 - MainThread - awscli.clidriver - DEBUG - Exiting with rc 255

[Errno 2] No such file or directory
```

Import less along with groff

Fixes #12549.

pkgs/top-level/python-packages.nix

···
       rsa
       pyasn1
       pkgs.groff
+      pkgs.less
     ];
 
     postInstall = ''