Add more constants for DRBD and change sync tests

This patch adds constants for the connection status, peer roles and disk
status, and it changes the rules for when the disk is considered as
“resyncing” - previously it was only for syncsource/synctarget, but
there are many other transient statuses which could be misinterpreted as
‘degraded’ (because they where not considered as resyncing, but the disk
is not consistent in these statuses).

Furthermore, cmdlib.py:WaitForSync determines if a device is syncing or
not based on sync_percent being not none. Not all DRBD resync statuses
offer a percent done, so if we are syncing but don't have a sync
percent, we'll report a zero sync percent (and no time estimate).

The patch also removes a few unused variables (is_sync_target,
peer_sync_target, is_resync) whose value doesn't make sense anymore with
the new sync rules.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>