[FLOC-3739] Failover acceptance test enabled

[itamarst]

Builds on #2546.

1. Enable the test on EBS, for now.
2. Fix bug in EBS code that was caught by this. (The API for starting nodes is only used by this test, so it's a bit of a hack...).
3. Fix some bugs in the test itself.
4. News entry since acceptance test means the implementation is hopefully going to continue to work, so we can tell people about it.

[james-w]

The two tests keep failing, and then get timed out by Jenkins before they can
print the errors. I've increased the timeout on those two jobs on Jenkins to see
what the failures are.

Note that with these changes if it now takes more than 45 minutes for
run_acceptance_loopback_on_AWS_CentOS_7_for_flocker.acceptance
and run_acceptance_loopback_on_AWS_Ubuntu_Trusty_for_flocker.acceptance
to pass then build.yaml will have to be updated for them.

Pushing to this branch will overwrite my cowboy, so you might want to
change the timeouts in build.yaml if you have to do more work on this
branch, even if they are reverted before merging to master because
a passing run takes less than 45 minutes.

[james-w]

Does assertIn work here?

[james-w]

✔️ I'm happy with this, thanks for the improvements.

Merge when you are happy and there are green tests, but I suspect there
may be work to do to get passing tests. A re-review of any changes needed
for that is at your discretion.