This removes all the complex downstream prefix / address selection
logic and reduces it to generating a random address within the selected
prefix and checking whether it satisfies a set of conditions. If the
conditions are not met, the process is retried (for a total of 10 times)
before eventually moving on to the next prefix.
The conditions that must be satisfied are:
- Address within prefix does not end in .0, .1, .255
- Address is not contained within a range of commonly used prefixes.
- Prefix does not overlap with another prefix on the system.
Test: TH
Change-Id: I4c53bcaa29c11cde4df99d5bcb834fde94961da4
b9cfcce30a