Add Read function for monitors

[ojongerius]

The monitor resource does not have read function (like resourceDatadogMonitorRead).

Having one should be faster, though I'm guessing most win is already enjoyed by the existing Exists resource.

[ojongerius]

Interesting. I was tempted to refactor this one a little; "warning" and "critical" are both schema.Typemap, and I know and have used schema.TypeSet before. What I like about using a Set is that the logic is handled in the schema, and its easier to see what's going on.

Currently the schema shows:

"critical": &schema.Schema{
                Type:     schema.TypeMap,
                Required: true,
            },

In the state file that will show up like so:

                       "attributes": {
                            "critical.#": "2",
                            "critical.notify": "@pagerduty",
                            "critical.threshold": "0",

The graph resource (using Set) state:

                        "attributes": {
                            "dashboard_id": "62086",
                            "description": "description for graph bar",
                            "id": "1438859136",
                            "request.#": "3",
                            "request.2366423942.query": "avg:system.mem.user{*}",
                            "request.2366423942.stacked": "false",
                            "request.3806987567.query": "avg:system.cpu.user{*}",
                            "request.3806987567.stacked": "false",

Anyway, it does not matter much which way to go. They hardest problem to solve is parsing the query before saving it. Also there are 2 types of monitors; metric and check based. I've raised #7 for this one.

[ojongerius]

This will be heaps easier after upstream API changes and work in #62 and #64

[ojongerius]

Revert to one simple monitor that does all the things, versus many small different monitors that do not do all the things in #73