Use only shared configuration lock for ComputeDRBDMap
Use only shared configuration lock for ComputeDRBDMap

This solves the following issue with failing cluster verify operations.
As this function declared to modify the configuration by taking an
exclusive lock, the configuration serial number changed during verify
operations, which in turn caused to fail other concurrently running
checks.

The mistake has been present in previous versions as well, but only
causes problems in 2.12 where each configuration write is explicitly
tracked.

Signed-off-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>
Reviewed-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>

bitemyapp/ganeti,andir/ganeti,mbakke/ganeti,grnet/snf-ganeti,leshchevds/ganeti,onponomarev/ganeti,ganeti/ganeti,bitemyapp/ganeti,dimara/ganeti,mbakke/ganeti,grnet/snf-ganeti,mbakke/ganeti,ganeti/ganeti,yiannist/ganeti,onponomarev/ganeti,dimara/ganeti,leshchevds/ganeti,yiannist/ganeti,leshchevds/ganeti,yiannist/ganeti,ganeti/ganeti,andir/ganeti,andir/ganeti