Allow users to specify cpu/memory_size when creating function, and
allow to update their values saved in function database.
This patch is api part of customized cpu/mem in qinling server and
is based on patch [0].
[0]: https://review.openstack.org/#/c/562507/
Story: 2001586
Task: 14366
Change-Id: I7a245f93a445a00c2722238d3f94d3a960f16af4