Bump BUILD_TIMEOUT for both Ironic drivers

BUILD_TIMEOUT is currently bumped from its default 196s to
300s for the Ironic agent_ssh deploy driver.  Enabling parallel
Tempest testing of the pxe_ssh fails periodically due to build
timeouts, likely due to concurrent deployments generating lots of
i/o.  This bumps the timeout for all Ironic jobs.

Change-Id: I638aa18c77d7a60e03df7bb8039dbd532cafa7ab
Merge "Bump BUILD_TIMEOUT for both Ironic drivers"