Implements blueprint cfg-filter-view

At the moment, if a module requires a configuration option from another
module, we do:

  CONF.import_opt('source.module', 'option_name')

but, in fact, all options from the imported module are available for
use.

The new ConfigFilter class makes it possible to enforce which options
are available within a module e.g. with

  CONF = cfgfilter.ConfigFilter(cfg.CONF)
  CONF.import_opt('foo', 'source.module')
  CONF.register_opt(StrOpt('bar'))

then the foo and bar options would be the only options available via
this CONF object while still being available via the global cfg.CONF
object.

Change-Id: Ie3aa2cd090a626da8afd27ecb78853cbf279bc8b

The cfg API is now available via the oslo-config library, so switch to
it and remove the copied-and-pasted version.

Add the 2013.1b3 tarball to tools/pip-requires - this will be changed
to 'oslo-config>=2013.1' when oslo-config is published to pypi. This
will happen in time for grizzly final.

Remove the 'deps = pep8==1.3.3' and 'deps = pyflakes' from tox.ini as
it means all the other deps get installed with easy_install which can't
install oslo-config from the URL.

Change-Id: I4815aeb8a9341a31a250e920157f15ee15cfc5bc

Created import_group method in cfg.py
Added testcases

Fixes bug #1107843

Change-Id: I427d4069dacbb6e586687370adbb08b5d50f7b63

Related to blueprint cfg-global-object

When using the global config object pattern, you often have modules
which define options that are referenced in other options.

So, for example if module A defined option 'foo' and module be needed
to reference that option, you might do:

  import A
  print CONF.foo

However, this makes it entirely unclear to the casual reader why
module A was imported.

Nova has a flags.DECLARE() function that helps with this problem
by allowing you to do:

  flags.DECLARE('foo', 'A')

The function simply imports module A and checks that the 'foo'
option is now defined in the global config object.

This is fine, but it is also implicit that this function applies
to the global config object. Instead, let's do the following:

  CONF.import_opt('foo', 'A')

Change-Id: I7b98f5be71068bbde70cc0eab991eaebb577de52