Warns a user when specifying a VTEP name.

This patch implements 'writeable' field of SingleAttr. If the field is
set to False, SingleAttr throws a newly defined UserException when a
user attempts to set a value to a read-only attribute such as VTEP
name and description, causing the command execution to abort and to go
back to the command line evaluation loop.

Please note that though this 'writeable' field hasn't been
implemented, many entities are specifying various fields 'writeable',
thus implementing this feature may have unforeseen impacts.

Fix MN-2510
Merge pull request #212 from midokura/dev/tomohiko/writeable-attr/20140707

Warns a user when specifying a VTEP name.
Make router name writable

Fixes: MN-2530
Merge pull request #215 from tomoe/mn-2530-master

Make router name writable
Merge pull request #209 from alexbikfalvi/master

Show the license warning upon startup if MN not licensed
Output license errors to STDERR

Fixes: MN-2533

Signed-off-by: Alex Bikfalvi <alex.bikfalvi@midokura.com>