define the default value for the `cwd`
define the default value for the `cwd`

Signed-off-by: Lai Jiangshan <b337eedf453972390e4695e362aeea2634ef4e0c@gmail.com>

jonboulle/specs,hqhq/runtime-spec,opencontainers/runtime-spec,hqhq/specs,wking/opencontainer-runtime-spec,wking/opencontainer-specs,ujuc/specs,laijs/specs,cloudfoundry-incubator/specs,vbatts/specs,opencontainers/specs