Export tags to cluster verify hooks

Export tags to cluster verify hooks

This patch export the cluster and node tags to the cluster verify hook
scripts. The tags are exported as a space-separated list, which allows
easy parsing from the shell (e.g. “for tag in $GANETI_CLUSTER_TAGS; do
...”) and therefore requires the previous “Don't allow spaces in tag
names” patch.

The patch also fixes a minor line length style problem.

Reviewed-by: ultrotter