Add '-E' CLI flag

Add '-E' CLI flag for overwriting single config options via CLI. For example:

`-E output.console.pretty: true` enabled console output with pretty printing
enabled.

CHANGELOG.asciidoc

@@ -52,6 +52,7 @@ https://github.com/elastic/beats/compare/v5.0.0-alpha4...master[Check the HEAD d
 - Add enable-setting to all output modules. {pull}1987[1987]
 - Command line flag -c can be used multiple times. {pull}1985[1985]
 - Add OR/AND/NOT to the condition associated with the processors. {pull}1983[1983]
+- Add '-E' CLI flag for overwriting single config options via command line. {pull}1986[1986]
 
 *Metricbeat*
 

libbeat/cfgfile/cfgfile.go

@@ -15,6 +15,7 @@ var (
 	// when this variable is created. See ChangeDefaultCfgfileFlag which should
 	// be called prior to flags.Parse().
 	configfiles = flagArgList("c", "beat.yml", "Configuration file")
+	overwrites  = common.NewFlagConfig(nil, nil, "E", "Configuration overwrite")
 	testConfig  = flag.Bool("configtest", false, "Test configuration and exit.")
 )
 
@@ -51,10 +52,23 @@ func Read(out interface{}, path string) error {
 // this method reads from the configuration file specified by the '-c' command
 // line flag.
 func Load(path string) (*common.Config, error) {
+	var config *common.Config
+	var err error
+
 	if path == "" {
-		return common.LoadFiles(configfiles.list...)
+		config, err = common.LoadFiles(configfiles.list...)
+	} else {
+		config, err = common.LoadFile(path)
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	err = config.Merge(overwrites)
+	if err != nil {
+		return nil, err
 	}
-	return common.LoadFile(path)
+	return config, nil
 }
 
 // IsTestConfig returns whether or not this is configuration used for testing

libbeat/common/config.go

@@ -1,8 +1,11 @@
 package common
 
 import (
+	"flag"
+
 	"github.com/elastic/go-ucfg"
 	"github.com/elastic/go-ucfg/cfgutil"
+	cfgflag "github.com/elastic/go-ucfg/flag"
 	"github.com/elastic/go-ucfg/yaml"
 )
 
@@ -34,6 +37,28 @@ func NewConfigWithYAML(in []byte, source string) (*Config, error) {
 	return fromConfig(c), err
 }
 
+func NewFlagConfig(
+	set *flag.FlagSet,
+	def *Config,
+	name string,
+	usage string,
+) *Config {
+	opts := append(
+		[]ucfg.Option{
+			ucfg.MetaData(ucfg.Meta{"command line flag"}),
+		},
+		configOpts...,
+	)
+
+	var to *ucfg.Config
+	if def != nil {
+		to = def.access()
+	}
+
+	config := cfgflag.ConfigVar(set, to, name, usage, opts...)
+	return fromConfig(config)
+}
+
 func LoadFile(path string) (*Config, error) {
 	c, err := yaml.NewConfigWithFile(path, configOpts...)
 	return fromConfig(c), err

libbeat/tests/system/test_base.py

@@ -39,6 +39,29 @@ def test_invalid_config(self):
         assert exit_code == 1
         assert self.log_contains("error loading config file") is True
 
+    def test_invalid_config_cli_param(self):
+        """
+        Checks CLI overwrite actually overwrites some config variable by
+        writing an invalid value.
+        """
+
+        self.render_config_template(
+            console={"pretty": "false"}
+        )
+
+        # first run with default config, validating config being
+        # actually correct.
+        proc = self.start_beat()
+        self.wait_until(lambda: self.log_contains("Setup Beat"))
+        proc.check_kill_and_wait()
+
+        # start beat with invalid config setting on command line
+        exit_code = self.run_beat(
+            extra_args=["-E", "output.console=invalid"])
+
+        assert exit_code == 1
+        assert self.log_contains("error unpacking config data") is True
+
     def test_config_test(self):
         """
         Checks if -configtest works as expected