Discovery: do not allow passing negative cluster size

[spzala]

When an etcd instance attempts to perform service discovery, if a
cluster size with negative value is provided, the etcd instance
will panic without recovery.

[spzala]

/cc @xiang90 @gyuho created this PR per last week's meeting. We could quickly fail if strconv.Atoi return any negative value but thought changing the type is a better solution. Thanks!

[codecov-io]

Codecov Report

Merging #11608 into master will decrease coverage by 1.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #11608      +/-   ##
==========================================
- Coverage   65.06%   64.01%   -1.05%     
==========================================
  Files         403      403              
  Lines       37645    37651       +6     
==========================================
- Hits        24494    24103     -391     
- Misses      11558    11940     +382     
- Partials     1593     1608      +15

Impacted Files	Coverage Δ
etcdserver/api/v2discovery/discovery.go	68.82% <100%> (ø)	⬆️
auth/options.go	47.5% <0%> (-45%)	⬇️
client/client.go	44.11% <0%> (-39.87%)	⬇️
client/keys.go	55.77% <0%> (-35.68%)	⬇️
proxy/httpproxy/director.go	61.42% <0%> (-18.58%)	⬇️
auth/store.go	54.63% <0%> (-16.94%)	⬇️
auth/jwt.go	66.29% <0%> (-11.24%)	⬇️
proxy/grpcproxy/register.go	69.44% <0%> (-11.12%)	⬇️
auth/simple_token.go	79.83% <0%> (-9.25%)	⬇️
clientv3/concurrency/mutex.go	62.16% <0%> (-5.41%)	⬇️
... and 24 more

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[xiang90]

lgtm