I want to add a mine function that gets the hostname of a minion.

`pillar/custom.sls`

```
mine_functions:
  custom:
    - mine_function: grains.get
    - nodename

```

I manually refresh the pillar data by running a 
`salt '*' saltutil.refresh_pillar`

and when running `salt '*' mine.get '*' custom` the output is as expected, showing a list of minions all with the nodename data underneath.

The issue is when I try to do thew following in a template file:

```
{%- set custom_nodes = [] %}
bootstrap.servers={% for host, custom in salt['mine.get']('role:foo', 'custom', expr_form='grain').items() %}
  {% do hosts.append(custom + ':2181') %}
{% endfor %}{{ custom_nodes|join(',') }}

```

I just get an empty space where my list of server nodenames should be.

I was hoping someone might be able to point out where I'm going wrong with this?