Add failure check into redis cluster implemenation

[zhduan]

I need the redis cluster support for my project asap. So I went ahead to try the work h4lflife submitted in #45 and made some changes to deal with redis failure scenario. It works well for my needs ( basic redis command get/set with a redis cluster by keep live option )

[agentzh]

Maybe you can cooperate with @pintsized in his lua-resty-redis-connector project or alike?

Please see https://groups.google.com/d/topic/openresty-en/0yr32RqZ8L0/discussion

[jzh800]

Whether to support with password access？
please give a simple usage demo with auth and clsuter!

[jzh800]

I have hacker the redis_cluster.lua file to support the password access.
now it is could work with cluster set or get.

maybe we need public the auth interface with redis_cluster?

local function get_redis_link(host, port, timeout)
local r = redis:new()

r:set_timeout(timeout)
r:connect(host, port)
r:auth("XXX-XXX")
	
return r

end

only change the redis host ip and port
Example:

local redis_cluster = require("redis_cluster")

local cluster_id = "test_cluster"

-- Subset of nodes within the cluster
local startup_nodes = {
{"127.0.0.1", 7004},
{"127.0.0.1", 7000},
{"127.0.0.1", 7001}
}

local opt = {
timeout = 100,
keepalive_size = 100,
keepalive_duration = 60000
}

local rc = redis_cluster:new(cluster_id, startup_nodes, opt)

rc:initialize()

local ok, err = rc:set("key1", "val1")
if not ok then
ngx.say("Unable to set key1: ", err)
else
ngx.say("key1 set result: ", ok)
end

local res, err = rc:get("key1")
if not res then
ngx.say("Failed to get key1: ", err)
else
ngx.say("key1:", res)
end

-- (same as above, slightly faster)
res, err = rc:send_cluster_command("get", "key1")
if not res then
ngx.say("Failed to get key1 with send_cluster_command: ", err)
else
ngx.say("key1 using send_cluster_command:", res)
end