Instance hooks now get an INSTANCE_TAGS environment variable, which contains a
space-delimited list of the affected instance's tags.

Also update the documentation to reflect the change.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>