Low: pgsql: support starting as Hot Standby

rep_mode=slave starts PostgreSQL as Hot Standby to connect to Primary.
This mode does not support Master/Slave configuration.
Low: pgsql: add check_wal_receiver parameter

add check_wal_receiver parameter to check wal_receiver process on monitor
and notify its status using "<resource name>-receiver-status" attribute
to check  whether PostgreSQL(Hot Standby) connects to primary.
The attribute shows status as "normal" or "ERROR".
Default is false.
Merge pull request #198 from t-matsuo/start-as-hs3

Start PostgreSQL as Hot Standby and check wal_receiver process