Update docker repo for Ubuntu 17.04

[changlan]

docker-ce package exists only in the edge branch.

[codecov]

Codecov Report

Merging #523 into master will decrease coverage by 2.58%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #523      +/-   ##
==========================================
- Coverage   67.48%   64.89%   -2.59%     
==========================================
  Files         198      198              
  Lines       12245    12245              
==========================================
- Hits         8264     7947     -317     
- Misses       3981     4298     +317

Impacted Files	Coverage Δ
core/modules/ether_encap.h	0% <0%> (-100%)	⬇️
core/modules/bpf.h	0% <0%> (-100%)	⬇️
core/modules/buffer.h	0% <0%> (-100%)	⬇️
core/modules/buffer.cc	4.34% <0%> (-95.66%)	⬇️
core/modules/ether_encap.cc	4.16% <0%> (-91.67%)	⬇️
core/modules/bpf.cc	0.72% <0%> (-52.79%)	⬇️
core/pktbatch.h	75% <0%> (-12.5%)	⬇️
core/modules/nat.cc	70.22% <0%> (-11.46%)	⬇️
core/utils/copy.h	54.9% <0%> (-8.83%)	⬇️
core/modules/drr.cc	74.05% <0%> (-7.55%)	⬇️
... and 4 more

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[sangjinhan]