Create a custom Django ORM object manager for VirtualMachine, Network,
Subnet, NetworkInterface, IPAddress and Volume object that can be used
in order to retrieve the objects that can be accessed by the user.

The resources that are accessible by the user include those that:
* are owned by the user
* are shared to a project the user is a member
* are public

The Port (NetworkInterface) resource needs special handling as it
connects two resources (VirtualMachine and Network) that may be owned by
different users in different projects. To overcome this issue, we make a
port accessible to a user, if is connected to a VirtualMachine or
Network that can be accessed by the user.

Signed-off-by: Christos Stavrakakis <stavr.chris@gmail.com>