envconsul [options] <command>, command runs in a subshell

### Envconsul version
0.13.0 and 0.13.1

### Command
```shell
./envconsul_0.12.1 -log-level=trace -kill-signal=SIGINT -exec-kill-signal=SIGINT -exec-kill-timeout=10s -upcase -secret _vault/aws/sts/internal-envconsul-example/ python3 -u ./program.py

./envconsul_0.13.0 -log-level=trace -kill-signal=SIGINT -exec-kill-signal=SIGINT -exec-kill-timeout=10s -upcase -secret _vault/aws/sts/internal-envconsul-example/ python3 -u ./program.py

./envconsul_0.13.1 -log-level=trace -kill-signal=SIGINT -exec-kill-signal=SIGINT -exec-kill-timeout=10s -upcase -secret _vault/aws/sts/internal-envconsul-example/ python3 -u ./program.py

```

### Debug output
```shell
2934949 pts/12 Ss 0:00 -bash
2946182 pts/12 Sl+ 0:00 \_ ./envconsul_0.12.1 -log-level=trace -kill-signal=SIGINT -exec-kill-signal=SIGINT -exec-kill-timeout=10s -upcase -secret _vault/aws/sts/internal-envconsul-example/ python3 -u ./program.py
2946191 pts/12 S 0:00 \_ python3 -u ./program.py

2948619 pts/17 Ss 0:00 -bash
2949574 pts/17 Sl+ 0:00 \_ ./envconsul_0.13.0 -log-level=trace -kill-signal=SIGINT -exec-kill-signal=SIGINT -exec-kill-timeout=10s -upcase -secret _vault/aws/sts/internal-envconsul-example/ python3 -u ./program.py
2949588 pts/17 S 0:00 \_ sh -c python3 -u ./program.py
2949589 pts/17 S 0:00 \_ python3 -u ./program.py

2946451 pts/16 Ss 0:00 -bash
2948386 pts/16 Sl+ 0:00 \_ ./envconsul_0.13.1 -log-level=trace -kill-signal=SIGINT -exec-kill-signal=SIGINT -exec-kill-timeout=10s -upcase -secret _vault/aws/sts/internal-envconsul-example/ python3 -u ./program.py
2948402 pts/16 S 0:00 \_ sh -c python3 -u ./program.py
2948403 pts/16 S 0:00 \_ python3 -u ./program.py
```

### Expected behavior
Expected 0.13.0 and 0.13.1 to behave like version 0.12.1

### Actual behavior
Versions 0.13.0 and 0.13.1 launch the python application in a subshell which can cause problem for signal handling and stopping the application to refresh credentials. If for some reason the application ignores the stop signal then envconsul will see the subshell stop but the application will still be running and envconsul will start a new instance of your application and you end up with an old application with old credentials and a new application with new fresh credentials.