config: cwd is a required default
config: cwd is a required default

After thorough discussion, even though a reasonable default is "/", for
platform independence, leave this up to the bundle author.

Also, by this variable being present it makes things explicit for the
runtime.

Signed-off-by: Vincent Batts <d01a4772f0cffd3f005a7d7f01c59cd104c7a5fe@hashbangbash.com>

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