When in the MMR + DNA setup the plugin configuration entry of DNA is modified,
the new attributes dnaRemoteBindMethod and dnaRemoteConnProtocol that are in
the shared configuration entry get erased by the plugin.

This happens when the entry is modified manually as well as when the plugin
config entry is modified when the plugin itself assigns a value to newly added
entry and updates the dnaNextValue attribute in the process.

This disables the dna range transfer process from a server that is not known to
the server making the request (e.g. M1 <-----> M2 <-----> M3 topology).

Steps to Reproduce:
1. Set up MMR and DNA with the topology from description
2. Set M1 and M2 to have only few remaining values above the threshold abd M3
with a lot of available space.
3. Add users to M1 to deplete the available values and fall beneath the
threshold, triggering the range transfer.

Actual results:
The server tries to get a new range from server M2, (probably) using
credentials from replication agreement, and is rejected. The server can't
contact the server M3 as it can't retrieve the attributes from the shared
entry.

logs:
[07/Mar/2014:14:18:44 -0500] dna-plugin - dna_pre_op: Passed threshold of 10
remaining values for range cn=account uids,cn=distributed numeric assignment
plugin,cn=plugins,cn=config. (6 values remain)
[07/Mar/2014:14:18:44 -0500] dna-plugin - dna_get_replica_bind_creds: Failed to
fetch replication agreement for range cn=Account
UIDs,ou=Ranges,dc=example,dc=com, server example.com, port 3389
[07/Mar/2014:14:18:44 -0500] dna-plugin - dna_request_range: Unable to retrieve
replica bind credentials.
[07/Mar/2014:14:18:44 -0500] dna-plugin - dna_request_range: Error sending
range extension extended operation request to server
<host>:2389 [error 53]

Expected results:
The server uses the method and protocol settings stored in the shared
configuration to contact server M3 and gets new range.