Change VNET_DOMAINNAME default and document it

VNET_DOMAINNAME must default to "eucalyptus.internal", so that
instances can resolve the short version of their private
hostname by default.

Fixes EUCA-5603
Swap CLC nameserver and VNET_DNS server

In order to properly resolve internal IP addresses, the
CLC's DNS server must be the first one in instances'
resolv.conf files.

Fixes EUCA-5600
Fix UDP DNS queries when CC/CLC are colocated

UDP queries could fail if the CLC and CC are on the same machine,
because the source address used for the CLC's response could
differ from the IP address to which the query was sent.  In this
case, the CC can simply advertise the router as the DNS server.

Fixes EUCA-5599
Implement split-horizon DNS

This changes several behaviors in the Eucalyptus DNS service:

1) "euca-x-x-x-x" hostnames were being resolved against both
public and private IPs, regardless of the Zone being searched.  Now
the FQDN will need to match for the search to succeed.

2) Public hostnames were resolving to public IPs for all queries.
Now they will be translated to the private IP when an instance
makes the query.

3) Private hostnames were resolvable from external systems.
This has been disabled.

Fixes EUCA-3116
Maybe fix for EUCA-6379
Merge parts of a13m/eucalyptus dns branch. Pull Request #24.
Adjust vnetwork.c to remove workaround related to UDP source address for DNS responses being wrong -- the DNS now listens on a canonical interface and will always respond w/ that address.
Mark related DNS fields as deprecated in the implementation.  Fix error logging
Merge branch 'dev/grze/EUCA-6379' into testing