dht: add DHTLOGLEVEL env var to select DHT log

This patch adds the support of DHTLOGLEVEL environment variable
to let the user choose OpenDHT log level when the deamon instance
is created.

Following level are available:
 - 0 : disable all logs
 - 1 : only errors
 - 2 : level 1 + warnings
 - 3 : level 2 + debugs

Change-Id: Ib55bdabdea2e3ac72c86fa78260a5e3d16b52179
Reviewed-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>