Merge lp://staging/~freyes/charm-helpers/ufw into lp://staging/charm-helpers

Dear Charmers,

Here I'm proposing a module to manage access to services using ufw, this is a helpful module to secure services that don't provide built-in mechanisms to control accesses (for example memcached).

Here are some examples on how the API is used:

- open SSH port for subnet 10.0.3.0/24:

  >>> from charmhelpers.contrib.network import ufw
  >>> ufw.enable()
  >>> ufw.grant_access(src='10.0.3.0/24', dst='any', port='22', proto='tcp')

- open service by name as defined in /etc/services:

  >>> from charmhelpers.contrib.network import ufw
  >>> ufw.enable()
  >>> ufw.service('ssh', 'open')

- close service by port number:

  >>> from charmhelpers.contrib.network import ufw
  >>> ufw.enable()
  >>> ufw.service('4949', 'close') # munin

Thanks,