12684: Support num_retries in PySDK client constructors

12684: Support num_retries in PySDK client constructors

This lets users set their preferred retry strategy once, rather than in
every call to execute(), which is error-prone. The default num_retries
is 10 because we expect most users to care more about eventual success
than responsiveness. See the added release notes for further discussion
and rationale.

Changes to the rest of the code are mostly about supporting this
consistently. Tests that relied on the old no-default-num_retries
behavior now specify that explicitly.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>