Load collector configuration

[masci]

What

Define a type to hold collector's configuration and a method to load config data from a YAML file.

Update:
Also provide an interface for configuration "providers" so that we can implement different procedures to retrieve configuration options from different locations.

Why

The collector Agent should be configured with global parameters as in v.5

How

We already have a system to search for and load configuration data from multiple sources and it is used by the check loader. This PR doesn't make any use of it for the following reasons:

1. The configuration format for any check is unknown at boot time because the Agent binary searches for and loads config data in a generic way, and only later it tries to load the checks corresponding to the configs found. In this case instead, we know in advance which are the global configuration parameters and we can easily provide a struct to hold them.
2. The configuration format for the checks heavily relies on the concept of Instance and we made specific choices to optimise that way, so the current loading system is not generic enough to support something that has nothing to do with instances.
3. The global configuration must be loaded before anything else, difficult to achieve with a generic approach

Update:
Anyway, we decided to provide a similar approach also to global configuration, see comments below.

[olivielpeau]

Just discussed with @masci: we could create an AgentConfigProvider interface that would roughly look like:

type AgentConfigProvider interface {
    Collect(*collector.Config) error
}

and have structs like FileAgentConfigProvider, EtcdAgentConfigProvider, etc that would implement it and populate the Config struct when Collect is called on them.

This would be "symmetric" with the way the check configs are loaded and probably make the logic clearer.

[olivielpeau]

Based on https://golang.org/pkg/go/build/#pkg-overview I think you need to name the file common.go instead of common_linux.go, otherwise the file will only be built on linux even if you list other platforms here.

[masci]

Good catch, will probably name it common_unix.go as they do in the standard lib, would leave common.go for stuff common to any OS

[olivielpeau]

One other small thing that came to my mind: should we rather have all the agent-config-related stuff in a different package than collector since it's related to the whole agent? Maybe a config or common package, or something of that kind.

[olivielpeau]

we should probably check the potential error Configure can return, and (for now, until we have more config providers) stop everything if there's an error

[olivielpeau]

Added in a few comments, other than that this looks 👍 📈

[olivielpeau]