fix: json suffix for json template from CLI

[lesam]

Closes #351

The server can reject jsonnet files due to security concerns, so don't pretend that our files are jsonnet templates.

[lesam]

Passes:

> ./influx template -f buckets.jsonnet
LABELS
+--------------+------------------+---------------+--------------------------------+---------+
| PACKAGE NAME |        ID        | RESOURCE NAME |          DESCRIPTION           |  COLOR  |
+--------------+------------------+---------------+--------------------------------+---------+
|   label-1    | 0000000000000000 |    label-1    | desc_1                         | #eee888 |
+--------------+------------------+---------------+--------------------------------+---------+
|                                                               TOTAL              |    1    |
+--------------+------------------+---------------+--------------------------------+---------+

BUCKETS
+--------------+------------------+---------------+-----------+--------------------------------+-------------+
| PACKAGE NAME |        ID        | RESOURCE NAME | RETENTION |          DESCRIPTION           | SCHEMA TYPE |
+--------------+------------------+---------------+-----------+--------------------------------+-------------+
|   rucket-1   | 0000000000000000 |   rucket-1    | 2h46m40s  | desc_1                         |  implicit   |
+--------------+------------------+---------------+-----------+--------------------------------+-------------+
|   rucket-2   | 0000000000000000 |   rucket-2    | 5h33m20s  | desc-2                         |  implicit   |
+--------------+------------------+---------------+-----------+--------------------------------+-------------+
|   rucket-3   | 0000000000000000 |   rucket-3    |  8h20m0s  | desc_3                         |  implicit   |
+--------------+------------------+---------------+-----------+--------------------------------+-------------+
|                                                                           TOTAL              |      3      |
+--------------+------------------+---------------+-----------+--------------------------------+-------------+

LABEL ASSOCIATIONS
+---------------+---------------+------------------+------------+------------------+
| RESOURCE TYPE | RESOURCE NAME |   RESOURCE ID    | LABEL NAME |     LABEL ID     |
+---------------+---------------+------------------+------------+------------------+
|    buckets    |   rucket-1    | 0000000000000000 |  label-1   | 0000000000000000 |
+---------------+---------------+------------------+------------+------------------+
|    buckets    |   rucket-2    | 0000000000000000 |  label-1   | 0000000000000000 |
+---------------+---------------+------------------+------------+------------------+
|    buckets    |   rucket-3    | 0000000000000000 |  label-1   | 0000000000000000 |
+---------------+---------------+------------------+------------+------------------+
|                                                      TOTAL    |        3         |
+---------------+---------------+------------------+------------+------------------+

Before this change:

> ./influx template -f buckets.jsonnet
Error: failed to summarize template: 422 Unprocessable Entity: template[0] from source(s) "buckets.jsonnet" had an issue: invalid encoding provided: jsonnet