bootstrap_system.sh fails to configure PostgreSQL 9 on some CentOS 7 systems

bin/bootstrap_system.sh relaxes the default PostgreSQL security rules to allow password-less operations during data load. It uses sed to manipulate the contents of the pg_hba.conf file, see the lines https://github.com/apache/impala/blob/master/bin/bootstrap_system.sh#L310

Unfortunately the sed patterns are too much system-dependent, which caused a setup failure when a CentOS 7 test platform was set up with PostgreSQL 9.2: the expected default content of pg_hba.conf did not match the script.

This failure broke the build in the dataload phase with the symptom:

23:36:49 dropdb: could not connect to database template1: FATAL:  Peer authentication failed for user "hiveuser"
23:36:49 createdb: could not connect to database template1: FATAL:  Peer authentication failed for user "hiveuser" 

This blocks automatic deployment of an Impala build on such a platform.