build(deps): bump github.com/coreos/etcd (#3199)

Bumps [github.com/coreos/etcd](https://github.com/coreos/etcd) from 3.3.13+incompatible to 3.3.15+incompatible.
- [Release notes](https://github.com/coreos/etcd/releases)
- [Changelog](https://github.com/etcd-io/etcd/blob/master/CHANGELOG-3.4.md)
- [Commits](https://github.com/coreos/etcd/compare/v3.3.13...v3.3.15)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

go.mod

@@ -15,7 +15,7 @@ require (
 	github.com/caddyserver/caddy v1.0.3
 	github.com/coredns/federation v0.0.0-20190818181423-e032b096babe
 	github.com/coreos/bbolt v1.3.2 // indirect
-	github.com/coreos/etcd v3.3.13+incompatible
+	github.com/coreos/etcd v3.3.15+incompatible
 	github.com/coreos/go-semver v0.2.0 // indirect
 	github.com/coreos/go-systemd v0.0.0-20190212144455-93d5ec2c7f76 // indirect
 	github.com/coreos/pkg v0.0.0-20180928190104-399ea9e2e55f // indirect