Fix issue with tags on instance creation

Commit 720f56c8 added the ability to specify tags when creating an
instance. The “tags” attribute of an instance object needs to be a set,
but the patch's code saved it as a list, causing breakage in other parts
of Ganeti. This patch changes the code to use TaggableObject.AddTag,
which has a nice side-effect of doing some verification (including max.
number of tags). Instance import was also broken (no “tags” attribute in
options).

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>