fix: failed to build api-server image (#462)
fix: failed to build api-server image (#462)

Some python dependencies changes cause the api-server image
failure. Some dependencies of kfp sdk need higher version then
the requirement.txt in `backend` directory. Since the second
stage is to compile the samples by using kfp sdk, directly
install the requirements.txt from `sdk/python` instead of
`backend`.

Signed-off-by: Yihong Wang <f520643bf8cff572a9eb593b2aa4ab3c892470e2@ibm.com>
kubeflow/kfp-tekton,kubeflow/kfp-tekton,kubeflow/kfp-tekton,kubeflow/kfp-tekton,kubeflow/kfp-tekton,kubeflow/kfp-tekton