OpsGenie2 Integration: teams() not working, if team derived from tags

We are currently moving from Slack Notifications to use Kapacitor with the OpsGenie Integration. We have the following TICKscript template "generic_value_alert.tick":
```
dbrp "telegraf"."retain30days"

// Which measurement to consume
var measurement string

// Optional list of group by dimensions
var groups = [*]

// Optional where filter
var whereFilter = lambda: TRUE

// function to compute the metric from the measurement
var metric lambda

// slack channel to post the alert to
var slackChannel = '#alertfeed'

// The threshold we want to alert on
var critLevel = 90

// message to post on alert
var message string

// name of the alert
var name string

var opteam = 'core'

var idVar = name + ' {{.Group}}'
var idTag = 'alertID'
var levelTag = 'level'
var messageField = 'message'
var durationField = 'duration'
var outputDB = 'chronograf'
var outputRP = 'autogen'
var outputMeasurement = 'alerts'
var triggerType = 'threshold'

var data = stream
|from()
.measurement(measurement)
.groupBy(groups)
.where(whereFilter)
|eval(metric)
.as('value')

var trigger = data
|alert()
.crit(lambda: "value" > critLevel)
.message(message)
.id(idVar)
.idTag(idTag)
.levelTag(levelTag)
.messageField(messageField)
.durationField(durationField)
.stateChangesOnly()
.slack()
.channel(slackChannel)
.opsGenie2()
.teams(opteam)

trigger
|eval(lambda: float("value"))
.as('value')
.keep()
|influxDBOut()
.create()
.database(outputDB)
.retentionPolicy(outputRP)
.measurement(outputMeasurement)
.tag('alertName', name)
.tag('triggerType', triggerType)

trigger
|httpOut('output')
```

and the following task definition in "disk_space_low.json":
```
{
"template-id": "generic_value_alert",
"vars": {
"name": {
"type": "string",
"value": "Disk: space used > 85%"
},
"measurement": {
"type": "string",
"value": "disk"
},
"metric": {
"type": "lambda",
"value": "\"used_percent\""
},
"critLevel" : {
"type": "int",
"value": 85
},
"groups": {
"type": "list",
"value": [
{
"type": "string",
"value": "host"
},
{
"type": "string",
"value": "path"
},
{
"type": "string",
"value": "opteam"
}
]
},
"opteam": {
"type": "string",
"value": "{{ index .Tags \"opteam\" }}"
},
"message": {
"type": "string",
"value": "*{{.Level}} - {{.ID}}*\nEvent Time: {{.Time}}\nDisk Path: {{ index .Tags \"path\" }}\nDisk Usage: {{ index .Fields \"value\" | printf \"%0.2f\" }}%\nTeam: {{ index .Tags \"opteam\" }}"
}
}
}
```
Note: We are using Telgraf and InfluxDB where each VM that we monitor has a global_tag name "opteam" assigned to it. This tag defines which Team in OpsGenie will receive alerts from the VM.

**Expected behaviour**
When an alert is fired (Disk space on a VM is low) the alert should be sent to OpsGenie with the `opteam` correctly assigned.

**Current behaviour**
In the message that we send to Slack we see that "{{ index .Tags \"opteam\" }}" is correctly displayed. However, in OpsGenie the team is not assigned correctly.

**Kapacitor OSS 1.5.5 configuration**
```
[opsgenie2]
enabled = true
api-key = "XXXXX-XXXXXX-XXXX-XXX"
url = "https://api.opsgenie.com/v2/alerts"
recovery_action = "close"
global = false
```

Start with the OpsGenie2 integration entry point used by `.opsGenie2().teams(opteam)` and reproduce the case from `generic_value_alert.tick`, `disk_space_low.json`, and the supplied `[opsgenie2]` configuration. Trace how the tag-derived `opteam` value is passed to the OpsGenie API, then verify that an alert assigns the expected team while Slack output remains correct.

Written by the indexing model from the issue text.