Enforce a minimum time between all IPMI commands

This patch enforces the min_command_interval option, which was added in
the previous patch, to ensure that any given BMC is never "poked" more
frequently than this interval, regardless of the calling method.

Closes-Bug: #1320513

Change-Id: Id3849aadf3908133a92157b3e96dd752610533e9
Merge "Enforce a minimum time between all IPMI commands"