Longer build timeout for Ironic agent driver

Does what it says; the agent driver is currently a bit slower than
the PXE driver, for a number of reasons.

Change-Id: I3f84f77899c163ccc7e5663784a8958f64f2c448

devstack-vm-gate.sh

@@ -136,6 +136,8 @@ EOF
             echo "IRONIC_DEPLOY_DRIVER=agent_ssh" >>localrc
             # agent driver doesn't support ephemeral volumes yet
             echo "IRONIC_VM_EPHEMERAL_DISK=0" >>localrc
+            # agent driver is a bit slow
+            echo "BUILD_TIMEOUT=300" >>localrc
         else
             echo "IRONIC_VM_EPHEMERAL_DISK=1" >>localrc
         fi