Honor WSK_CONFIG_FILE if variable is set (#1054)
Honor WSK_CONFIG_FILE if variable is set (#1054)

The OpenWhisk CLI (wsk) allows to set the WSK_CONFIG_FILE to point to
the configuration file. wskdeploy does not honor this environment
variable and it only relies on the --config flag. This change allows to
use the environment variable (if present), making it consistent with the
wsk CLI behaviour.

Fixes #809
Fixes #436
pritidesai/incubator-openwhisk-wskdeploy,openwhisk/openwhisk-wskdeploy,pritidesai/incubator-openwhisk-wskdeploy,openwhisk/openwhisk-wskdeploy,pritidesai/incubator-openwhisk-wskdeploy,openwhisk/openwhisk-wskdeploy,pritidesai/incubator-openwhisk-wskdeploy,pritidesai/incubator-openwhisk-wskdeploy,openwhisk/openwhisk-wskdeploy,pritidesai/incubator-openwhisk-wskdeploy,pritidesai/incubator-openwhisk-wskdeploy,pritidesai/incubator-openwhisk-wskdeploy