During cluster init, set the default hypervisor to be used for instances.
Ensure that the default hypervisor belongs to the set enabled hypervisors
for this cluster. Also fix a small bug with setting the default enabled
hypervisor list.

Reviewed-by: imsnah

Reviewed-by: iustinp

Reviewed-by: iustinp

ssconf will become write-only from ganeti-masterd's point of view,
therefore all settings in there need to go into the main configuration
file.

Reviewed-by: iustinp

Future patches will add even more variables to the cluster config.
Adding more parameters wouldn't make the function easier to use and
it doesn't make sense to pass them to another function, as it's
only done once in bootstrap.py on cluster initialization.

Reviewed-by: iustinp

This patch completely  gets rid of fping
 - replace all fping invocations with TcpPing calls
 - update documentation accordingly.
 - associated cleanups (use constant for localhost IP, use more sensible
   defaults for TcpPing and _use_ those)

Reviewed-by: iustinp

Reviewed-by: schreiberal

We currently require that hostnames are FQDN not short names
(node1.example.com instead of node1). We can allow short names as long
as:
  - we always resolve the names as returned by socket.gethostname()
  - we rely on having a working resolver

These issues are not as big as may seem, as we only did gethostname() in
a few places in order to check for the master; we already required
working resolver all over the code for the other nodes names (and thus
requiring the same for the current node name is normal).  The patch
moves some resolver calls from within execution path to the checking
path (which can abort without any problems). It is important that after
this patch is applied, no name resolving is called from the execution
path (LU.Exec() or other code that is called from within those methods)
as in this case we get much better code flow.

This patch also changes the functions for doing name lookups and
encapsulates all functionality in a single class.

The final change is that, by requiring working resolver at all times, we
can change the 'return None' into an exception and thus we don't have to
check manually each time; only some special cases will check
(ganeti-daemon and ganeti-watcher which are not covered by the
generalized exception handling in cli.py). The code is cleaner this way.

Reviewed-by: imsnah

  - add an Update method on the ConfigWriter class;
  - make the AddInstance method not log disks for diskless types (helps with testing)
  - implement limited testing for the ConfigWriter with focus on the new Update method