Introduce a Luxi call for GetTags

This changes from submitting jobs to get the tags (in cli scripts) to
queries, which (since the tags query is a cheap one) should be much
faster.

The tags queries are already done without locks (in the generic query
paths for instances/nodes/cluster), so this shouldn't break tags query
via gnt-* list-tags.

On a small cluster, the runtime of gnt-cluster/gnt-instance list tags
more than halves; on a big cluster (with many MCs) I expect it to be
more than 5 times faster. The speed of the tags get is not the main
gain, it is eliminating a job when a simple query is enough.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>