Add missing create() method on SecurityGroupRule object

The nova.objects.SecurityGroupRule class was missing the
create() method, so we add it here, and while doing so,
ensure that the id attribute is marked as read-only.

Partial-Blueprint: compute-manager-objects-juno
Change-Id: I4584e33d9d163a7dce01b01d205f2f43824e544f
Correct seconds of a day from 84400 to 86400

Correct a day to 86400 seconds which is used by API rate limit config
Closes-bug: #1357152

Change-Id: Ieceacd00f02b6ed27032cc0ac2f55be8b976a070
Merge "Add missing create() method on SecurityGroupRule object"
Merge "Correct seconds of a day from 84400 to 86400"