Adjust Docker file and rpmbuild.sh for new rpm name.

Also restructure Dockerfile rpmbuild.sh. Now
pgpool-II-33-pg92/rpmbuild.sh and pgpool-II-33-pg93/rpmbuild.sh are
identical. Also pgpool-II-33-pg92/Dockerfile and
pgpool-II-33-pg93/Dockerfile are almost same except the version number
ENV command.
Let rpm command to download rpm and avoid using wget.
Change rpm dist tag to rhel6.
Add new rpm builder for CentOS6/RHEL6 and CentOS7/RHEL7.

CentOS6/pgpool-II-33: for pgpool-II 3.3.* and PostgreSQL 9.2, 9.3, 9.4
CentOS6/pgpool-II-34: for pgpool-II 3.4.* and PostgreSQL 9.2, 9.3, 9.4
CentOS7/pgpool-II-33: for pgpool-II 3.3.* and PostgreSQL 9.2, 9.3, 9.4
CentOS7/pgpool-II-34: for pgpool-II 3.4.* and PostgreSQL 9.2, 9.3, 9.4
Remove "exit" statement which accidentally remained for debugging.