double the screen sleep timeout

HP Cloud 1.1 is quite slow per core. So much so that grenade is
often failing to start services because bash in a screen session
isn't ready in 1.5s. Double this to 3s to try to decrease failures
in the gate.

We should instead do this via some readiness check to decrease
races.

Change-Id: I2b066a687916742e966190f00b5b06a795d4a014
Merge "double the screen sleep timeout"