Issue 466 partial

Follow-on to commit r2858.  ADSContext#updateServer is called by Installer#updateADS (via registerOrUpdateServer) in the case the server entry already exists in ADS. With this commit, updateServer checks 'cn=instance keys,cn=admin data' for an entry with the instance key public-key certificate of the server being (re)registered (or otherwise updated and the instance-key certificate server property is supplied). If the entry exists, it is used. Otherwise, a new entry is created.  The new routine ADSContext#registerInstanceKeyCertificate contains the code common to registerServer and updateServer.

Note that the updateServer is not fully tested since I cannot figure out how to re-register an existing instance (i.e., unregister, but not uninstall, an instance; then re-register the instance so the existing instance-key entry is reused).

M      src/ads/org/opends/admin/ads/ServerDescriptor.java
M      src/ads/org/opends/admin/ads/ADSContext.java