rapi: make tags query not use jobs

Currently the rapi tags query implementation is similar to the command
line one: it submits OpGetTags jobs. This not good, since this being an
API it can be used a lot and can pollute the job queue with many such
trivial jobs.

This patch converts it to use either queries (for nodes/instances) or
direct read from ssconf (for the cluster case). For ssconf, we added a
function to the ssconf.SimpleStore class for reading the tags.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>