Wait for IPv6 address to be available at boot

[sargun]

Wait for the IPv6 Address to be usable before considering it done set…ting up

Previous to this, we would give up an IPv6 immediately to the container, but
due to the way SLAAC works, it takes a moment for the actual IPv6 address to
be usable.

This chooses a more conservative option, and waits for the address to become
usable before giving it to the user. On average, it takes about 5 seconds,
but I've seen as high as 20 seconds, when EC2 is having issues.

[coveralls]

Pull Request Test Coverage Report for Build 1933

• 0 of 70 (0.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 24.726%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
executor/runtime/docker/docker.go	0	8	0.0%
vpc/allocate/setup_container_linux.go	0	62	0.0%

Totals
Change from base Build 1903:	-0.1%
Covered Lines:	2434
Relevant Lines:	9844

---

💛 - Coveralls

[codecov]

Codecov Report

Merging #168 into master will decrease coverage by 0.13%.
The diff coverage is 3.38%.

@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
- Coverage    33.8%   33.66%   -0.14%     
==========================================
  Files          64       64              
  Lines        7765     7803      +38     
==========================================
+ Hits         2625     2627       +2     
- Misses       4822     4856      +34     
- Partials      318      320       +2

Impacted Files	Coverage Δ
vpc/allocate/allocate_network.go	0% <ø> (ø)	⬆️
vpc/allocate/setup_container_linux.go	0% <0%> (ø)	⬆️
executor/runtime/docker/docker.go	50.55% <33.33%> (-0.1%)	⬇️