uptime doesn't take TZ into account

[dikt1338]

I figured, that the "uptime sensor" uses uptime -s but later adds "Z" (=Zulu =CET) to the timestamp value before sending via mqtt.
According to my testing this leads to wrong values as uptime -s is always local (sorry can't find a man entry to prove that. This is according to some blog posts I found and is confirmed by my own tests)

I changed the script like that - maybe this could be considered for further releases?

def check_uptime(format):
    if format == 'timestamp':
        full_cmd = "uptime -s"
        tz_cmd = "date +%z"
        tz_str = subprocess.Popen(tz_cmd, shell=True, stdout=subprocess.PIPE).communicate()[0].decode('utf-8').strip()**
        timestamp_str = subprocess.Popen(full_cmd, shell=True, stdout=subprocess.PIPE).communicate()[0].decode('utf-8').strip()
        timestamp = datetime.strptime(timestamp_str, '%Y-%m-%d %H:%M:%S')
        iso_timestamp = timestamp.isoformat() + tz_str  # Append correct offset to indicate `local` time

Basically, I use
data +%z
to get the time zone offset of the current installation to send the correct timestamp