Provide config option to cap events per stack

Previously users could write to the events table infinitely. With this
change Heat will automatically prune some events if the maximum is
reached. The behavior can be disabled by setting max_events_per_stack to
zero.

Change-Id: I4fb2fefbd9bcd10ce1767ddf58a870206a6482a1
Fixes-Bug: #1209492
Merge "Provide config option to cap events per stack"