properly reset external cpi usage in integration tests

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>
simplified external_cpi_spec.rb

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>
handle NotSupported/NotImplemented/VMNotFound errors in external cpis

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>