Refactor export and idxmgmt handling.

[simitt]

At the moment one idxmgmt.Supporter and one ilm.Supporter can be defined for one beat, with the following interfaces:

idxmgmt.Supporter:

// Supporter provides index management and configuration related services
// throughout libbeat.
// The BuildSelector is used by the output to create an IndexSelector. The
// index selector will report the per event index name to be used.
// A manager instantiated via Supporter is responsible for instantiating/configuring
// the index throughout the Elastic Stack.
type Supporter interface {
	// Enalbed checks if index management is configured to configure templates,
	// ILM, or aliases.
	Enabled() bool

	// ILM provides access to the configured ILM support.
	ILM() ilm.Supporter

	// TemplateConfig returns the template configuration used by the index supporter.
	TemplateConfig(withILM bool) (template.TemplateConfig, error)

	// BuildSelector create an index selector.
	// The defaultIndex string is interpreted as format string. It is used
	// as default index if the configuration provided does not define an index or
	// has no default fallback if all indices are guarded by conditionals.
	BuildSelector(cfg *common.Config) (outputs.IndexSelector, error)

	// Manager creates a new manager that can be used to execute the required steps
	// for initializing an index, ILM policies, and write aliases.
	Manager(client ESClient, assets Asseter) Manager
}

ilm.Supporter and ilm.Manager:

// Supporter implements ILM support. For loading the policies and creating
// write alias a manager instance must be generated.
type Supporter interface {
	Mode() Mode
	Alias() Alias
	Policy() Policy
	Manager(h APIHandler) Manager
}

// Manager uses an APIHandler to install a policy.
type Manager interface {
	Enabled() (bool, error)

	EnsureAlias() error

	// EnsurePolicy installs a policy if it does not exist. The policy is always
	// written if overwrite is set.
	// The created flag is set to true only if a new policy is created. `created`
	// is false if an existing policy gets overwritten.
	EnsurePolicy(overwrite bool) (created bool, err error)
}

Using these interfaces as is does not allow for having multiple templates nor multiple ILM policies for one beat. This is a requirement for APM though.

During some discussion with @urso we agreed on moving forward by allowing for having multiple ILM supporter per idxmgmt supporter, and abstracting away the template handling. This can be achieved by removing following from the idxmgmt.Supporter interface:

	// ILM provides access to the configured ILM support.
	ILM() ilm.Supporter

	// TemplateConfig returns the template configuration used by the index supporter.
	TemplateConfig(withILM bool) (template.TemplateConfig, error)

The main changes arising from this is related to the export commands, as they were using those methods.
Therefore this PR addresses two main points:

• Change interface definition for idxmgmt.Supporter to allow for multiple ilm supporter and more flexible template handling
• Reuse supporter logic for export template and export ilm-policy cmds and refactor export package to reuse and align more logic. Exporting templates and ilm-policy also allows to specify a --dir option now so one can write them to a file in a specified directory.

TODO:

• Update changelog.

[simitt]

@urso this is ready for another round of review.

[urso]

nice. Makes me wonder if it would make sense to have this as a positional parameter instead of a CLI flag. E.g. what happens when we introduce multiple templates support, where to write to if no dir is given?

[simitt]

If no dir is given, we write to stdout.

[urso]

Jenkins, please test this.

[urso]

Jenkins, test this.

[simitt]

jenkins, test this.