Alerta origin() don't render templated value

Example TICK fragment:

```
.alert()
.alerta()
.event('{{ index .Tags "host" }}')
.origin('{{ index .Tags "host" }}')
```

Output:
``` yaml
event: hostname.tld
origin: {{ index .Tags "host" }}
```

Kapacitor don't replace `'{{ index .Tags "host" }}'` to tag "host" value and send raw string.
Other alerta options templated correctly.

Kapacitor 1.5.0-1, Ubuntu 18.04

Start by reproducing the provided TICK fragment on Kapacitor 1.5.0-1 and compare how the templated event and origin values are rendered. Done means origin resolves the host tag value instead of sending the raw template, while the other Alerta options continue to render correctly.

Written by the indexing model from the issue text.