[18.03 backport] use a custom grpc dialer when managers are joining

[thaJeztah]

backport of #2802 for 18.03

Minor conflict in import statements, due to #2750 not being in the 18.03 branch

diff --cc manager/state/raft/util.go
index da44dca6,1a49f76c..00000000
--- a/manager/state/raft/util.go
+++ b/manager/state/raft/util.go
@@@ -1,10 -1,10 +1,15 @@@
  package raft
  
  import (
++<<<<<<< HEAD
++=======
+       "context"
+       "net"
++>>>>>>> 3a531f45... use a custom grpc dialer when managers are joining
        "time"
  
 +      "golang.org/x/net/context"
 +
        "github.com/docker/swarmkit/api"
        "github.com/docker/swarmkit/manager/state"
        "github.com/docker/swarmkit/manager/state/store"

[codecov]

Codecov Report

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The diff coverage is 100%.

@@              Coverage Diff               @@
##             bump_v18.03    #2828   +/-   ##
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  Coverage               ?   61.58%           
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  Files                  ?      134           
  Lines                  ?    21834           
  Branches               ?        0           
==============================================
  Hits                   ?    13446           
  Misses                 ?     6935           
  Partials               ?     1453

[thaJeztah]

marking this "WIP" because there's some comments on the original PR, and we should have it merged in master first

[wk8]

LGTM once the original PR is merged