Allow users to set an arbitrary place for ValueBox

[KazuCocoa]

ValueBox currently creates a directory in a user-specific place. For example, /home/user/.local/share/appium-lg-webos-driver-nodejs on Linux. The /home/user/.local/share is hard coded place by envPath. We'd like to manage driver specific files in a place such as /home/user/project/path instead of the hardcoded place by envPath.

For example when we set LG_WEBOS_DRIVER_VALUE_BOX=/home/user/project/path, the value box will create files in LG_WEBOS_DRIVER_VALUE_BOX/appium-lg-webos-driver-nodejs instead of /home/user/.local/share/appium-lg-webos-driver-nodejs.

[KazuCocoa]

cc @jlipps @boneskull

[boneskull]

ValueBox is @appium/strongbox or will be soon, so I'll implement it there.