Pause DRBD sync for OS install if not wait_for_sync

When wait_for_sync is set to False in LUInstanceCreate, Ganeti lets DRBD sync
in the background while performing the rest of the installation steps,
including OS installation.

However, OS installation is a very disk-intensive task that intereferes badly
with the background I/O caused by DRBD's initial sync. To this end, we pause
the background sync before OS installation and unpause it afterwards, which
yields a significant speed boost for OS installation. The following should be
noted:

a) The user has requested not to wait for sync, i.e. the instance will be
   non-redundant for an unspecified interval anyway and delaying this by a
   couple of minutes is not a big compromise.

b) This approach is also followed during disk wiping.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
[iustin@google.com: simplify an if check]
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>