engine_life_check_timeout support

Since https://github.com/openstack/heat/commit/d7ae961fcae5e0e75bf232606a6878d2414dfeef
got merged, heat engine has now a distributed stack lock using the
database to avoid race conditions when multiple engines are deployed.

This patch aims to add a new flag to configure the RPC timeout for the
engine liveness check that is used for stack locking.

Change-Id: Iad7958225651e2b52844b5d942f6630bbe9e86fa
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>