Add support of the terminal client for the server

[git-hulk]

This PR implements the simple client for the controller server:

# Show help
$ ./_build/kvctl --help

# Create namespace
$ ./_build/kvctl create namespace test-ns

# List namespaces
$ ./_build/kvctl list namespaces

# Create a cluster in the namespace 
$ ./_build/kvctl create cluster test-cluster --nodes 127.0.0.1:6666,127.0.0.1:6667 -n test-ns

# List clusters in the namespace
$ ./_build/kvctl list clusters -n test-ns

# Get cluster in the namespace
$ ./_build/kvctl get cluster test-cluster -n test-ns

# Migrate slot from source to target
$ ./_build/kvctl migrate slot 123 --target 1 -n test-ns -c test-cluster