Fix a bug concerning TCP port release

Commit f396ad8c returns the TCP port used by DRBD disk back to the
TCP/UDP port pool using AddTcpUdpPort().

However, AddTcpUdpPort() writes the config on every invocation,
using _WriteConfig(). This causes two problems:

 * it causes critical errors logged by VerifyConfig(), after the DRBD
   disk removal, and until the actual instance removal.
 * if the code following AddTcpUdpPort() fails, the port is already
   returned back the pool, which causes the port to have duplicates
   (inconsistent config).

AddTcpUdpPort() is invoked in three cases:

 * during InstanceRemove() through _RemoveDisks().
 * during InstanceSetParams() in case of disk removal.
 * during InstanceSetParams() through _ConvertDrbdToPlain().

This commit fixes the problem by removing the _WriteConfig() call from
AddTcpUdpPort(), delegate it to Update() via the
TemporaryReservationManager and ensure AddTcpUdpPort() precedes
Update().

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
[iustin@google.com: small comments adjustements]
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>