Fix preInit logic when mixed disk situations exist. (#4904)

When servers are started simultaneously across multiple
nodes or simulating a local setup, it can happen such
that one of the servers in setup reaches a following
situation where it observes

 - Some servers are formatted
 - Some servers are unformatted
 - Some servers are offline

Current state machine doesn't handle this correctly, to fix
this situation where we have unformatted, formatted and
disks offline we do not decisively know the course of
action. So we wait for the offline disks to change their state.

Once the offline disks change their state to either one of these
states we can decisively move forward.

  - nil (formatted disk)
  - errUnformattedDisk
  - Or any other error such as errCorruptedDisk.

Fixes #4903

appveyor.yml

@@ -13,10 +13,11 @@ clone_folder: c:\gopath\src\github.com\minio\minio
 environment:
   GOVERSION: 1.8.3
   GOPATH: c:\gopath
+  GOROOT: c:\go18
 
 # scripts that run after cloning repository
 install:
-  - set PATH=%GOPATH%\bin;c:\go\bin;%PATH%
+  - set PATH=%GOPATH%\bin;c:\go18\bin;%PATH%
   - go version
   - go env
   - python --version