Fix setting telemetry update period to default
Fix setting telemetry update period to default

- previously if not config file was not found, the update period was
  never set to default and the reading was done continuously (without sleep)
- now the default will be always set even without any config file
- this also defines minimum allowed update period to be >=5 seconds

Signed-off-by: Ondrej Fabry <237151fbd58e417e6d72d788598ce96c14d7b9e5@cisco.com>

VladoLavor/vpp-agent,VladoLavor/vpp-agent,ondrej-fabry/vpp-agent,VladoLavor/vpp-agent,ondrej-fabry/vpp-agent,ondrej-fabry/vpp-agent