Fix the default behaviour for networking of servers that are being
created. There are two settings controlling this behaviour:

* CYCLADES_DEFAULT_SERVER_NETWORKS: This setting contains list of
  networks to connect a newly created server to, if the user has not
  specified them explicitly in the POST /server API call. Each member of
  the list may be a network UUID, a tuple of network UUIDs,
  "SNF:ANY_PUBLIC_IPV4" [any public network with an IPv4 subnet
  defined], "SNF:ANY_PUBLIC_IPV6 [any public network with only an IPV6 subnet
  defined],  or "SNF:ANY_PUBLIC" [any public network].

  Access control and quota policy are enforced, just as if the user had
  specified the value of CYCLADES_DEFAULT_SERVER_NETWORKS in the content
  of the POST /call, after processing of "SNF:*" directives.

  Default value: ["SNF:ANY_PUBLIC"]

* CYCLADES_FORCED_SERVER_NETWORKS: This setting contains a list of
  networks which every new server will be forced to connect to,
  regardless of the contents of the POST /servers call, or the value of
  CYCLADES_DEFAULT_SERVER_NETWORKS.  Its format is identical to that of
  CYCLADES_DEFAULT_SERVER_NETWORKS.

 No access control or quota policy are enforced.  The server will get
 all IPv4/IPv6 addresses needed to connect to the networks specified in
 CYCLADES_FORCED_SERVER_NETWORKS, regardless of the state of the
 floating IP pool of the user, and without allocating any floating IPs.

 Default value: ["SNF:ANY_PUBLIC_IPV6"]

Also this commit changes how API handles requests that specify a public
network without specifying a floating IP address: If the request does
not contain the 'fixed_ip' attribute, the server will try to
automatically use one of the free floating IP addresses of the user
that are allocated from the specified network.