Added support for external etcd in cache-server

[gman0]

Summary

This PR adds etcd flags to cache-server command. It's now possible to point the cache-server to an external etcd cluster:

$ ./bin/cache-server \
  --etcd-servers 127.0.0.1:8379 \
  --etcd-cafile ~/ws/etcd/certs/ca.pem \
  --etcd-certfile ~/ws/etcd/certs/client.pem \
  --etcd-keyfile ~/ws/etcd/certs/client-key.pem

Once running, it can be then referenced from the sharded-test-server like so:

UNSAFE_E2E_HACK_DISABLE_ETCD_FSYNC=true NO_GORUN=1 \
  ./bin/sharded-test-server \
  --cache-kubeconfig=/home/rvasek/ws/kcp/standalone-cache-server/cache.kubeconfig \
  --quiet \
  --v=2 \
  --log-dir-path="/home/rvasek/ws/kcp/.kcp" \
  --work-dir-path="/home/rvasek/ws/kcp" \
  --shard-run-virtual-workspaces=false \
  --shard-feature-gates=WorkspaceMounts=true,CacheAPIs=true,WorkspaceAuthentication=true \
  --proxy-feature-gates=WorkspaceMounts=true,CacheAPIs=true,WorkspaceAuthentication=true \
  --number-of-shards=2

What Type of PR Is This?

/kind feature

Related Issue(s)

Fixes #3822

Release Notes

cache-server: added etcd flags
sharded-test-server: added --cache-kubeconfig flag to use an external cache-server

[ntnn]

/lgtm
/approve

[kcp-ci-bot]

LGTM label has been added.

Details

Git tree hash: 22601de6f4db01b1c615b0791f2213d83b5bbdc3

[kcp-ci-bot]

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This pull-request has been approved by: ntnn

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[ntnn]

/test pull-kcp-test-e2e-sharded

flake #3811 :(

[cnvergence]

@ntnn eh, let's give it another shot then

[neolit123]

Suggested change

	cacheServerKubeconfig := flag.String("cache-kubeconfig", "", "Path to a kubeconfig of an external cache-server. If empty, a dedicated cache-server is started.")
	cacheServerKubeconfig := flag.String("cache-kubeconfig", "", "Path to a kubeconfig of an external cache server. If empty, a dedicated cache server is started.")

[neolit123]

Suggested change

	func start(proxyFlags, shardFlags []string, logDirPath, workDirPath string, numberOfShards int, quiet bool, cacheSyntheticDelay time.Duration, cacheServerConfigPath string) error {
	func start(proxyFlags, shardFlags []string, logDirPath, workDirPath string, numberOfShards int, quiet bool, cacheSyntheticDelay time.Duration, cacheServerKubeconfig string) error {

or keep the name the flag variable the same for consistency.

[neolit123]

should the external and local etcd flags be somewhat mutually exclusive?