Plugins/Metadata: fix resolution for ipv4-in-ipv6 addresses like ::ffff:127.0.0.1

Hi,

while connecting with an old client (bcfg2-1.2.3) to an new server (bcfg2-1.3.1+) I noticed that the special form for embedding an IPv4 address into the IPv6 address space (like ::ffff:130.133.110.115) could not be resolved by gethostbyaddr leading to this backtrace:

    Unexpected Authentication Failure
    Traceback (most recent call last):
      File "/usr/lib/python2.6/dist-packages/Bcfg2/SSLServer.py", line 221, in parse_request
        if not self.authenticate():
      File "/usr/lib/python2.6/dist-packages/Bcfg2/SSLServer.py", line 211, in authenticate
       password, client_address)
     File "/usr/lib/python2.6/dist-packages/Bcfg2/Server/Core.py", line 1164, in authenticate
       address)
     File "/usr/lib/python2.6/dist-packages/Bcfg2/Server/Plugins/Metadata.py", line 1313, in AuthenticateConnection
       client = self.resolve_client(address)
     File "/usr/lib/python2.6/dist-packages/Bcfg2/Server/Plugins/Metadata.py", line 1058, in resolve_client
       cname = socket.gethostbyaddr(address)[0].lower()
    error: Address family not supported by protocol
    ::ffff:130.133.110.115 - - [27/Jun/2013 17:34:38] code 401, message Unauthorized

This is fixed with this little patch.