[warden] Proxy detection compatibility fix
[warden] Proxy detection compatibility fix

  The '-v' test in Bash is too new (requires Bash 4.2+), revert to old
trick of testing whether a variable is unset.

Change-Id: I2628849f35ab646542a63bcb540beabbf6137149

cloudfoundry-incubator/garden-linux,tsaleh/garden,tangfeixiong/garden,tangfeixiong/garden,shashidharatd/garden-linux,shashidharatd/garden-linux,cloudfoundry-incubator/garden-linux,cloudfoundry-incubator/garden,shashidharatd/garden-linux,cloudfoundry-incubator/garden-linux,savaki/garden,savaki/garden,shashidharatd/garden-linux,cloudfoundry-incubator/garden,tsaleh/garden