Updated eucadb.py to use /usr/pgsql-9.1/bin/pg_dump instead of /usr/bin/pg_dump

When running pg_dump before, the following would occur:

```
# pg_dump -c -o -h /var/lib/eucalyptus/db/data -p 8777 -U root eucalyptus_walrus
pg_dump: server version: 9.1.9; pg_dump version: 8.4.13
pg_dump: aborting because of server version mismatch
```

Need to use  /usr/pgsql-9.1/bin/pg_dump instead of /usr/bin/pg_dump for this command to execute successfully.