grid: Increase default promise timeout to 20 seconds

This is the default configuration of most of the deployed slapos.core cases,
so follow it here.

slapos/grid/slapgrid.py

@@ -79,7 +79,7 @@ COMPUTER_PARTITION_STOPPED_STATE = 'stopped'
 SLAPGRID_SUCCESS = 0
 SLAPGRID_FAIL = 1
 SLAPGRID_PROMISE_FAIL = 2
-PROMISE_TIMEOUT = 3
+PROMISE_TIMEOUT = 20
 
 COMPUTER_PARTITION_TIMESTAMP_FILENAME = '.timestamp'
 COMPUTER_PARTITION_LATEST_BANG_TIMESTAMP_FILENAME = '.slapos_latest_bang_timestamp'