Log file naming hardcode the time and extensions separators

In [log file name](https://github.com/awslabs/flowgger/blob/master/src/flowgger/utils/rotating_file.rs#L128) the separator with the datetime string and the extension strings are hardcoded.

So, using another char than "-" or no extension doesn't work.

```
file_path = "var/output/logs/fwsyslog.log"
file_rotation_timeformat = ".%Y-%m-%d-%H-%M"
file_rotation_time = 60

# want fwsyslog.2021-12-17-15-06.log
# get fwsyslog-.2021-12-17-15-39.log
```

```
file_path = "var/output/logs/fwsyslog"
file_rotation_timeformat = ".%Y-%m-%d-%H-%M"
file_rotation_time = 60

# want fwsyslog.2021-12-17-15-06
# get fwsyslog-.2021-12-17-15-39.log
```

Update ```new_file.set_file_name(&format!("{}-{}.{}"```
- The datetime separator must be removed and put in the config string.
- The ".{}" suffix should only be appended if the filename contains a suffix

Start at src/flowgger/utils/rotating_file.rs#L128 and inspect how new_file.set_file_name formats rotated paths. Compare the two configuration examples in the issue, including custom datetime separators and paths with or without extensions. Done means the generated names match the requested examples without adding a hardcoded separator or suffix.

Written by the indexing model from the issue text.