[backport] 2.x Add Environment Variable Support Flag (--allow-env)

[talevy]

backport of: #4963

for merging into 2.x and 2.3 branches

[ph]

LGTM,

~/e/logstash git:pr/5003 ❯❯❯ bin/logstash -e 'input { generator  { message => "PH is ${QUALITY}" count => 1 }} filter { mutate {}}'                                                                                                                                                                                       ✱
Settings: Default pipeline workers: 4
Pipeline main started
{
      "sequence" => 0,
    "@timestamp" => 2016-04-04T20:20:38.232Z,
      "@version" => "1",
          "host" => "sashimi",
       "message" => "PH is ${QUALITY}"
}
Pipeline main has been shutdown
^CSIGINT received. Shutting down the agent. {:level=>:warn}
stopping pipeline {:id=>"main"}
~/e/logstash git:pr/5003 ❯❯❯ bin/logstash -e 'input { generator  { message => "PH is ${QUALITY}" count => 1 }} filter { mutate {}}' --allow-env                                                                                                                                                                           ✱
fetched an invalid config {:config=>"input { generator  { message => \"PH is ${QUALITY}\" count => 1 }} filter { mutate {}}output { stdout { codec => rubydebug } }", :reason=>"Cannot evaluate `${QUALITY}`. Environment variable `QUALITY` is not set and there is no default value given.", :level=>:error}
~/e/logstash git:pr/5003 ❯❯❯ export QUALITY="awesome"                                                                                                                                                                                                                                                                   ⏎ ✱
~/e/logstash git:pr/5003 ❯❯❯ bin/logstash -e 'input { generator  { message => "PH is ${QUALITY}" count => 1 }} filter { mutate {}}' --allow-env                                                                                                                                                                           ✱
Settings: Default pipeline workers: 4
Pipeline main started
{
      "sequence" => 0,
    "@timestamp" => 2016-04-04T20:21:01.036Z,
      "@version" => "1",
          "host" => "sashimi",
       "message" => "PH is awesome"
}
Pipeline main has been shutdown
^CSIGINT received. Shutting down the agent. {:level=>:warn}
stopping pipeline {:id=>"main"}

[andrewvc]

@talevy something is wrong with the I18N lookup, but it's otherwise good

    --allow-env                   translation missing: en.logstash.runner.flag.allow_env (default: false)

[ph]

@andrewvc good catch I haven't checked that :( oops.

[andrewvc]

LGTM

[elasticsearch-bot]

Tal Levy merged this into the following branches!

Branch	Commits
2.3	d158fe7
2.x	7d86a61

[talevy]

Jarvis merged into 2.x and 2.3. closing.