### Summary

Some properties, like `KONG_SSL_CERT` and `KONG_SSL_CERT_KEY` are
arrays and users can specify many. Vaults didn't work in this scenario:

For example below didn't work before:
```
CERT_1=$(<cert1.crt) \
KEY_1=$(<key1.key) \
CERT_2=$(<cert2.crt) \
KEY_2=$(<key2.key) \
KONG_SSL_CERT_KEY="{vault://env/key-1},{vault://env/key-2}" \
KONG_SSL_CERT="{vault://env/cert-1},{vault://env/cert-2}" \
kong prepare --vv
```

There were also erroneous warning in logs like because of bad array handling:

```
[warn] 680#0: [kong] vault.lua:1475 error caching secret reference {vault://env/cert-1}: bad value type
```

This fixes those.

Signed-off-by: Aapo Talvensaari <[email protected]>