[18.03] migrate to CircleCI 2.0

[thaJeztah]

This migrates the 18.03 branch to use CircleCI 2.0, making sure that CI can still run after August. Backports:

• Fix possible data race in manager/state/store/memory_test.go #2537 Fix possible data race in manager/state/store/memory_test.go
• Bump Go to 1.10.3 #2636 Bump Go to 1.10.3
• Convert the circle config file to v2 format, as Circle v1 is deprecated #2659 Convert the circle config file to v2 format, as Circle v1 is deprecated
• Just use the default GOPATH in CircleCI #2690 Just use the default GOPATH in CircleCI

git checkout -b 18.03-circle-2.0 upstream/bump_v18.03 
git cherry-pick -s -S -x 0ad63eaff3733cf1175d667c5e2ce6e8660089ba
git cherry-pick -s -S -x 19114b466a3bf1a538e40d641f809d9d3bf74515
git cherry-pick -s -S -x 7fc0c4f3a35bd7d9e8c26f6d86acf7e2718eb778

cherry-picks were clean

[thaJeztah]

ping @cyli @anshulpundir @dperny PTAL

[thaJeztah]

Hm, interesting; possibly a change we don't have yet, and causing failure on Go 1.10?

# github.com/docker/swarmkit/manager/state/store
manager/state/store/memory_test.go:2046: loop variable c captured by func literal
make: *** [vet] Error 1
Exited with code 2

[thaJeztah]

Found it; #2536 - fixed by #2537 - adding it to the list

[codecov]

Codecov Report

Merging #2696 into bump_v18.03 will decrease coverage by 0.3%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           bump_v18.03    #2696      +/-   ##
===============================================
- Coverage         61.6%   61.29%   -0.31%     
===============================================
  Files              134      134              
  Lines            21795    21795              
===============================================
- Hits             13426    13359      -67     
- Misses            6929     6997      +68     
+ Partials          1440     1439       -1

[thaJeztah]

👍 perfect; it's green now 🎉

[cyli]

Thanks for backporting this @thaJeztah! LGTM

[dperny]

Very important, CI would have broken on this branch.