[general] Minor fixes for extending django-ipam

[anurag-ks]

Small changes which were required while extending django-ipam into openwisp-ipam.

[coveralls]

Pull Request Test Coverage Report for Build 134

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.544%

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Totals
Change from base Build 127:	0.0%
Covered Lines:	437
Relevant Lines:	439

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💛 - Coveralls

[anurag-ks]

@nemesisdesign thankyou for the review, I had to add the the app_label because of this error - RuntimeError: Model class django_ipam.models.Subnet doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS., I will try to look for correct solution for this.

[nemesifier]

@anurag-ks take a look at what Rohith is doing, ask him questions if needed

[anurag-ks]

@nemesisdesign another thing I noticed is that when I add the OrgMixin or rather the organization field to the models, the test cases under openwisp-ipam fail. Since the organization field cannot be empty.