Add env[SWIFTCLIENT_INSECURE]

Add env[SWIFTCLIENT_INSECURE] as default of --insecure option.
If set to 'true', allow to access insecure keystone server.
The name follows 'NOVACLIENT_INSECURE' in novaclient.

Change-Id: I322674eba9c07e6def97bce339815fa15191a92d
Merge "Add env[SWIFTCLIENT_INSECURE]"