Remove unused launch_time from instance

'launch_time' is passed around as part of instance properties when an
instance is being built, however it is not a column in the DB nor is it
even used anywhere.

It was also unnecessarily copied around in various tests.

This patch removes all references to it.

Change-Id: I2300cd6b8539d15bbcd35ab4c00ab99d4be13eb5
Merge "Remove unused launch_time from instance"