cooker: use time.strftime() instead of spawing sh with date

cooker: use time.strftime() instead of spawing sh with date

time.strftime() defaults to using time.localtime(), use that instead
of spawning "sh -c 'date +<FMT>'"

(Bitbake rev: ec1f2773b4811b3beb52710120bb5a9b215c174e)

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>