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