Added support to retrieve wifi ssid from environment
Added support to retrieve wifi ssid from environment

The wifi connect/disconnect unit test function gets
the ssid interface from the environment variable
TEST_WIFI_SSID.

Signed-off-by: Yossi Eliaz <2e304003926f02b38235c0ef612770eac907ffb7@resin.io>

resin-io-modules/network_manager,resin-io-modules/network_manager