allow creation of container and subnet of the same prefix and length

[miesi]

Now a days /24 prefixes/allocations are quiet common and configured as /24 on router interfaces.

currently there are two bad options:

• do two /25 subnets in a /24 container (but this breaks "document in DIM what is configured on the irb")
• do a /23 container with a comment that it is just a /24 and do a /24 subnet in it (breaks "document reality")

write tests
change the database constraint
modify code to allow this only for containers and subnets

[TheRealBecks]

I found the exception in this line

@miesi What do you think we want to achieve? Do we want to be able to...:

1. Always be able to create subnets of the same size as the parent container?
2. Only be able to create a subnet of the same size as the parent container if the parent container has the attribute ORIGIN-LIR?

[miesi]

having two objects at the same prefix/length breaks the show command which shall only display one object e.g. show ipblock
breaks also the api.