Prevent pcp_recovery_node from recovering "unused" status node.

Prevent pcp_recovery_node from recovering "unused" status node.

This allowed to try to recover a node without configuration data,
which leads to variety of problems. See discussion:
https://www.pgpool.net/pipermail/pgpool-general/2018-March/006021.html
for more details.

Also I fixed pgpool_recovery function so that it quotes an empty
string argument with double quotes. Without this, the argument is
treated as if it does not exist, which was the source of the complain
from the user.