burnin: introduce instance alive checks
This patch adds instance alive checks after most start operations. The
check is done in a custom way:
- the instance is expected to have an http server up and running
- and it should server the '/hostname.txt' resource containing the
hostname of the instance
This allows checking that:
- creation is working OK
- start after failover (and in the future migrate) is ok
- rename works correctly
By default, the check is disabled since one needs a custom OS for this
check.
The patch also fixes a wrong variable name from a previous burnin patch.
Reviewed-by: ultrotter