ct-worker: Fix local testing

[rozbb]

Previously, following the openssl bash line in the "local deployment section" was failing because it uses a hard-coded shard, regardless of the certificate's not-after timestamp.

This PR:

1. Replaces the bash line with a Python script that picks the shard correctly
2. Extends our dev shards into 2027
3. Extends local testing dev shards into 2027
4. Updates the script that creates shards to have better error handling and documentation

Fixes #160

[bwesterb]

LGTM

@cjpatton, please have a look.

[cjpatton]

Nice!