Support the TLS connection when communicating with etcd cluster. We added a configuration item to custom the certificate verification. Whether to setup TLS connection or not depends on the endpoints' scheme, for instance, when endpoints are:
```
etcd:
host:
- "https://127.0.0.1:2379"
- "https://127.0.0.1:3379"
```
APISIX will originate TLS connection automatically, and the Server Name Indication extention will be set by the endpoint host (`127.0.0.1` in above case). Note by default APISIX will verify the certificate, close the verification in configuration explicitly if you want to bypass it.
```
etcd:
tls:
verfiy: false
```
* bugfix: create the etcd object in `xpcall`, it may fail, the return values of `etcd.new` should be `res, err`.
fix issue: #2310
1. The old process, if creating etcd fails, etcd data will no longer be synchronized. We need to create the etcd object in xpcall.
2. the return value should be res, err of etcd.new.
* test: old test case is unstable, should delete some checkpoint which is wrong.
