pkg-export-kubernetes: Allow outputting to a file
pkg-export-kubernetes: Allow outputting to a file

Allow users to write the kubernetes manifest to a file.

Signed-off-by: Zeeshan Ali <f7a8e49325fe4a238df42df4f7e30abcfdad87c6@kinvolk.io>

habitat-sh/habitat,georgemarshall/habitat,habitat-sh/habitat,nathenharvey/habitat,georgemarshall/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,georgemarshall/habitat,nathenharvey/habitat,rsertelon/habitat,habitat-sh/habitat,nathenharvey/habitat,nathenharvey/habitat,georgemarshall/habitat,habitat-sh/habitat,habitat-sh/habitat,nathenharvey/habitat,habitat-sh/habitat,georgemarshall/habitat,georgemarshall/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,rsertelon/habitat,georgemarshall/habitat,rsertelon/habitat,georgemarshall/habitat