Support resolving other meta data variables inside the meta stanza

Hi all,
I've noticed that interpolation feature seems not working correctly on [meta stanza] but on application parameter: following job file example is very similar what normally is done for software parameters configuration.
As suggestion perhaps could help to activate variables resolution also in meta stanza.

If filing a bug please include the following:

### Nomad version
until 0.8.3

### Operating system and Environment details
tested on linux darwin

### Nomad Server logs
There are not particular logs

### Nomad Client logs
There are not particular logs

### Job file

 job "MY_JOB" {

 	datacenters = ["dc1"]

 	type = "batch"

 	id = "MY_JOB"

 	name = "MY_JOB"

 	meta {

 		RELEASE_VERSION = "2018.1.1.0"

 		MY_HOME = "/opt/mysoftware-${NOMAD_META_RELEASE_VERSION}"

 		MY_BIN = "${NOMAD_META_MY_HOME}/bin"

 		MY_CFG = "${NOMAD_META_MY_HOME}/cfg"

 		MY_EXECUTABLE = "${NOMAD_META_MY_BIN}/myExecutable" // could be an option

 		MY_CFG_FILE = "${NOMAD_META_MY_CFG}/mySoftware.conf" // could be an option

 	}

 

 	group "MyGroup" {

 		task "MyTask" {

 			driver = "raw_exec"

 			config = {

				command = "${NOMAD_META_MY_BIN}/myExecutable"

 				args = [

 					"--notWorks", "${NOMAD_META_MY_CFG_FILE}",

 					"--thisWorks", "/opt/mysoftware-${NOMAD_META_RELEASE_VERSION}/cfg/mySoftware.conf"

 				]

 			}

 		}

 	}

 }