yum-validator: add retry/timeout to subscription-manager subprocess calls

`subscription-manager` can fail for repo-override commands due to HTTP
errors when contacting the remote candlepin server. This change adds a
retry with expanding timeout for `subscription-manager` calls to try
to account for these failures.
Merge pull request #502 from dobbymoodge/handle_rhsm_errors

yum-validator: add retry/timeout to subscription-manager subprocess calls

I've tested this already, and need to cherry-pick it downstream