Create function entry_key() to create a ContentEntry ndb.Key.
Create function entry_key() to create a ContentEntry ndb.Key.

This will make sharding with ContentNamespaceShard easier by hiding the
intermediary entity in this function and create_entry(). No functional change
here.

R=vadimsh@chromium.org
BUG=swarming:75

Review URL: https://codereview.appspot.com/63280043

madecoste/swarming,pombreda/swarming,madecoste/swarming,madecoste/swarming,pombreda/swarming,luci/luci-py,luci/luci-py,pombreda/swarming,luci/luci-py,luci/luci-py,pombreda/swarming,madecoste/swarming