cluster: implement environment variable parsing based on a schema

At this moment we use environment variables just as strings. It leads to incompatibility with tarantool:

It fails for tt:

$ TT_CONFIG_ETCD_ENDPOINTS="http://127.0.0.1:2379" TT_CONFIG_ETCD_PREFIX="/default" tt start app1
   ⨯ can't collect instance information for app1: error loading instance "router-a-001" configuration from config "/home/user/work/tmp/vshard_template/instances.enabled/app1/config.yaml": unable to parse cluster config from file: failed to parse a configuration data as a cluster config: failed to unmarshal ClusterConfig: yaml: unmarshal errors:
  line 3: cannot unmarshal !!str `http://...` into []string

But if you run tarantool with the same environment variables, then everything works fine.

So we need to re-implement logic from the tarantool to parse environment variables in the same way:

https://github.com/tarantool/tarantool/blob/b8d733df8ce0c5da137fa2bba7c185b2fd877935/src/box/lua/config/utils/schema.lua#L1594-L1781

Reproduce the failure with the documented TT_CONFIG_ETCD_ENDPOINTS and TT_CONFIG_ETCD_PREFIX values while loading instances.enabled/app1/config.yaml. Read Tarantool's schema.lua environment-variable parsing implementation at the linked lines, then verify that tt start app1 accepts the same values and loads the cluster configuration successfully.