Allow setting custom lowstate data on startCommand

[lukasraska]

Currently, SaltClient.startCommand sets fixed lowstate data sent towards API, which limits the usage in certain way.

Following usecase specifies the needs for this.
As Job cache method prep_jid can accept passed_jid via lowstate data, users / systems might be able to craft custom JID before actual job execution, thus bypassing several race conditions when JID wasn't known prior to actual execution.