Discover the docker registry port only once

The method of autodiscovering the docker registry is
slow, but only needs to be performed once. Rather than
discovering it on each spawn/snapshot/etc, we may simply
perform this discovery during host init.

Change-Id: Ic719d71c410104f2ae0343103c59bed497958318
Closes-Bug: 1286171
Stub get_meminfo to prevent reading /proc/meminfo

We don't want to read from /proc/meminfo while running tests. It
isn't pretty and it only works on Linux.

Creates a new stubs module and updates test_hostinfo to use
it for its hostinfo.get_meminfo stub. The TestHostInfoCase
tests now use Stubs rather than assignment operators.

Change-Id: Ifbf47e0c0faebf6b52139c2b38b673a7b9310d2f
Closes-Bug: 1284990
Merge "Stub get_meminfo to prevent reading /proc/meminfo"
Merge "Discover the docker registry port only once"