remove restrictions on null ipam driver

[clinta]

This PR removes the restrictions on the null driver, providing a workaround for the issue discussed in moby/moby#20547.

We would like to be able to have a custom network driver assign a specific IP to containers using the --ip parameter, but this cannot be done on a network created without --subnet. But we cannot create a network using the null driver with a specific subnet.

With this change we can create networks using our own network driver, and the null ipam driver, with specific subnets configured allowing containers to be configured with specif IP addresses.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "null-allow-subnet" [email protected]:clinta/libnetwork.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@1ba8194). Click here to learn what that means.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##             master    #2078   +/-   ##
=========================================
  Coverage          ?   40.56%           
=========================================
  Files             ?      138           
  Lines             ?    22175           
  Branches          ?        0           
=========================================
  Hits              ?     8995           
  Misses            ?    11857           
  Partials          ?     1323

Impacted Files	Coverage Δ
ipams/null/null.go	39.39% <66.66%> (ø)

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