Add a ‘tags’ field to instance and node listing

Currently there isn't any easy way to list all nodes or instance and
their tags; you have to query each node in turn, or list all the tags
via something like “gnt-cluster search-tags '.*'”. Of course, this is
not optimal.

The patch adds a new fields to “gnt-instance list” and “gnt-node list”
called ‘tags’, that will list the tags of the object in comma-separated
form. This field will be empty if there are no tags (when using a
separator this output can still be parsed by other scripts).

At opcode level, there is a new fields called ‘tags’ that returns a
(python) list of the object tags.

Reviewed-by: ultrotter