Add flags for enabling TLS for store

[agalitsyn]

Add capability to use TLS for communication with store, for example with secure etcd

You can check it out by using following steps:

• Rebuild stolon bins from this branch
• Quickly setup secure etcd using this tool
• Run stolon bins using proper values to flags, like:

$ cat Procfile 
sentinel: ./bin/stolon-sentinel --cluster-name stolon-cluster --store-backend etcd --store-endpoints localhost:8080 --store-cert $GOPATH/src/github.com/coreos/etcd/hack/tls-setup/certs/proxy1.pem --store-key $GOPATH/src/github.com/coreos/etcd/hack/tls-setup/certs/proxy1-key.pem --store-cacert $GOPATH/src/github.com/coreos/etcd/hack/tls-setup/certs/ca.pem
keeper0: ./bin/stolon-keeper --data-dir data/postgres0 --id postgres0 --cluster-name stolon-cluster --pg-repl-username repluser --pg-repl-password replpassword --pg-su-username postgres --pg-su-password qweqwe --pg-bin-path /usr/lib/postgresql/9.4/bin --store-backend etcd --store-endpoints localhost:8080 --store-cert $GOPATH/src/github.com/coreos/etcd/hack/tls-setup/certs/proxy1.pem --store-key $GOPATH/src/github.com/coreos/etcd/hack/tls-setup/certs/proxy1-key.pem --store-cacert $GOPATH/src/github.com/coreos/etcd/hack/tls-setup/certs/ca.pem
proxy: ./bin/stolon-proxy --cluster-name stolon-cluster --port 25432 --store-backend etcd --store-endpoints localhost:8080 --store-cert $GOPATH/src/github.com/coreos/etcd/hack/tls-setup/certs/proxy1.pem --store-key $GOPATH/src/github.com/coreos/etcd/hack/tls-setup/certs/proxy1-key.pem --store-cacert $GOPATH/src/github.com/coreos/etcd/hack/tls-setup/certs/ca.pem

[agalitsyn]

Sorry, I didn't recognize integration tests, so I will update them too

[sgotti]

@agalitsyn Sorry for the delay and thanks for this PR!

I haven't had time to deeply test it but, since it'll continue working without tls store communication, it LGTM.

Can you please squash it in an unique commit?

[kayrus]

@agalitsyn I'd argue on this PR. I suppose it is better to update libkv to 0.2.1 (it contains docker/libkv@e7f836c)
And pass ClientTLSConfig pointer instead of bunch of strings.

[agalitsyn]

@kayrus sure, go ahead, we can close this PR

[sgotti]

@agalitsyn @kayrus libkv has been updated to v0.2.1 in #131. Sorry but I'm missing if someone is going to work on this (to directly pass ClientTLSConfig)?

[BarnabyShearer]

v0.2.1 also includes docker/libkv@d635a8e which reverted out the actual implementation for ClientTLSConfig, leaving this pull request as the best working option atm.

[sgotti]

Superseded by #208.