Intermittent issue with format="GOOGLE_APPLICATION_CREDENTIALS{{ .data.private_key_data }}"

When I use `format=` in the `secret` stanza for Variable interpolation, sometimes EnvConsul sets the incorrect value. This problem is fairly easy to reproduce.

In this case, I am reading a dynamic GCP service account key from Vault. Vault returns 3 fields in data (full key is truncated as ):
```
vault read -format=json gcp/key/$role
{
"request_id": "5e493ddb-b794-5bf0-9a44-14bdceb1dcdc",
"lease_id": "gcp/key/viewer/1XXN6zn7CizhXluJPGthrH1v",
"lease_duration": 50,
"renewable": true,
"data": {
"key_algorithm": "KEY_ALG_RSA_2048",
"key_type": "TYPE_GOOGLE_CREDENTIALS_FILE",
"private_key_data": "ewo"
},
"warnings": null
}
```
The application expects GOOGLE_APPLICATION_CREDENTIALS to contain the value of `private_key_data`. This can be achieved by specifying: `format="GOOGLE_APPLICATION_CREDENTIALS{{ .data.private_key_data }}"`
However, sometimes it gets set as `KEY_ALG_RSA_2048` or `TYPE_GOOGLE_CREDENTIALS_FILE`.

### Envconsul version
```
envconsul v0.9.1 (b5e928a7)
```

### Configuration
testgcp1.hcl file
```hcl
sanitize=true
upcase=true
log_level="debug"
pid_file="/tmp/envconsul.pid"

vault {
# Using VAULT_ADDR
renew_token = false

ssl {
enabled = false
verify = false
}
}
exec {
command = "/Users/kawsark/code/local/envconsul/app-wrapper.sh"
splay = "5s"
}

secret {
format="GOOGLE_APPLICATION_CREDENTIALS{{ .data.private_key_data }}"
no_prefix = false
path = "gcp/key/viewer"
}
```

### Command
```shell
envconsul -config testgcp1.hcl
```

### Debug output
Provide a link to a GitHub Gist containing the complete debug output by running
with `-log-level=debug`.
Please see: [https://gist.github.com/kawsark/f659cf6644cbac94527ae7f062dbdb63](https://gist.github.com/kawsark/f659cf6644cbac94527ae7f062dbdb63)

### Expected behavior
`GOOGLE_APPLICATION_CREDENTIALS` environment variable should be populated with Base64 encoded Google service account. In the Gist below, when it works, GOOGLE_APPLICATION_CREDENTIALS is shown as below (full key is truncated as ):
[https://gist.github.com/kawsark/f659cf6644cbac94527ae7f062dbdb63](https://gist.github.com/kawsark/f659cf6644cbac94527ae7f062dbdb63)
```shell
Loop # 0
Checking for GOOGLE_APPLICATION_CREDENTIALS
ewo=
```

### Actual behavior
Although it works most of the time, sometimes `GOOGLE_APPLICATION_CREDENTIALS` environment variable contain a value of `KEY_ALG_RSA_2048` or `TYPE_GOOGLE_CREDENTIALS_FILE`. In the Gist below, when it does not work, GOOGLE_APPLICATION_CREDENTIALS is set to KEY_ALG_RSA_2048 on line 102
[https://gist.github.com/kawsark/f659cf6644cbac94527ae7f062dbdb63#file-envconsul-txt-L100](https://gist.github.com/kawsark/f659cf6644cbac94527ae7f062dbdb63#file-envconsul-txt-L100)
```shell
Loop # 0
Checking for GOOGLE_APPLICATION_CREDENTIALS
KEY_ALG_RSA_2048
```

### Steps to reproduce
1. Using a root or admin token, setup a GCP secrets engine as below:
```
vault login
export GOOGLE_CLOUD_PROJECT=""
export role="viewer"

vault secrets enable gcp
vault secrets tune -default-lease-ttl=50s -max-lease-ttl=60s gcp
vault write gcp/config credentials=@gcp-root-service-account.json
vault write gcp/roleset/$role project="${GOOGLE_CLOUD_PROJECT}" secret_type="service_account_key" bindings=-< envconsul.policy
path "gcp/key/$role" {
capabilities = ["read"]
}
EOF

vault policy write envconsul envconsul.policy
vault token create -policy=envconsul -ttl=24h
export VAULT_TOKEN=
# Or:
# export VAULT_TOKEN=$(vault token create -ttl=24h -policy=envconsul | grep 'token.*s\.' | awk '{print $NF}')
```

3. Download example application and invoke EnvConsul
Example application script provided here: [https://gist.github.com/kawsark/74a87420dd9a3a1d9a0c7d4276d30aa9](https://gist.github.com/kawsark/74a87420dd9a3a1d9a0c7d4276d30aa9)
```
wget https://gist.githubusercontent.com/kawsark/74a87420dd9a3a1d9a0c7d4276d30aa9/raw/9b72a1242aa36f3743a883c632d686e0666208c7/app-wrapper.sh
chmod +x app-wrapper.sh
envconsul -config testgcp1.hcl
```

### References
Are there any other GitHub issues (open or closed) that should be linked here?