YUM updateinfo parser should tolerate errata without an issued timestamp

[sr-rossanderson]

Some valid YUM/RHUI updateinfo metadata does not provide the timestamp in the exact field Patchman expects.

Patchman currently expects an erratum issued timestamp, but metadata may instead provide an appropriate updated timestamp.

Expected behaviour:

Use issued when present and fall back to updated where necessary, while still validating the resulting timestamp.

Impact:

Improves compatibility with RHUI and other valid YUM updateinfo producers.