Description of problem:
engine-setup fails to upgrade postgres from 9.2 to 9.5 if the system wide locale differs from system wide locale at first installation time

engine-setup initializes its DB in en_US.UTF-8 while postgres own DB is initialized in system wide locale.
pg_upgrade cannot change DB locale on the fly on upgrades.

So if the user changed the system wide locale before running engine-setup to update the system from 4.1 to 4.2 and so from postgres 9.2 to postgres 9.5, engine-setup will fail with:

2017-12-20 15:53:42,967+0100 DEBUG otopi.plugins.ovirt_engine_setup.ovirt_engine.db.dbmsupgrade plugin.execute:926 execute-output: ('/opt/rh/rh-postgresql95/root/usr/bin/postgresql-setup', '--upgrade', '--upgrade-from=postgresql') stderr:
 * upgrading from 'postgresql.service' to 'rh-postgresql95-postgresql.service'
 * Upgrading database.
ERROR: pg_upgrade tool failed
ERROR: Upgrade failed.
 * See /var/lib/pgsql/upgrade_rh-postgresql95-postgresql.log for details.

2017-12-20 15:53:42,968+0100 DEBUG otopi.transaction transaction._prepare:66 exception during prepare phase
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/otopi/transaction.py", line 62, in _prepare
    element.prepare()
  File "/usr/share/ovirt-engine/setup/ovirt_engine_setup/engine_common/postgres.py", line 716, in prepare
    raiseOnError=True,
  File "/usr/lib/python2.7/site-packages/otopi/plugin.py", line 931, in execute
    command=args[0],
RuntimeError: Command '/opt/rh/rh-postgresql95/root/usr/bin/postgresql-setup' failed to execute

And in /var/lib/pgsql/upgrade_rh-postgresql95-postgresql.log something like:
Performing Consistency Checks
-----------------------------
Checking cluster versions                                   ok
Checking database user is the install user                  ok
Checking database connection settings                       ok
Checking for prepared transactions                          ok
Checking for reg* system OID user data types                ok
Checking for contrib/isn with bigint-passing mismatch       ok
Checking for invalid "line" user columns                    ok
Creating dump of global objects                             ok
Creating dump of database schemas
  engine
  ovirt_engine_history
  postgres
  template1
                                                            ok

lc_collate values for database "postgres" do not match:  old "en_US.UTF-8", new "de_DE.UTF-8"
Failure, exiting


Version-Release number of selected component (if applicable):
4.2 beta

How reproducible:
100%

Steps to Reproduce:
1. setup oVirt-engine 4.1 with a specific locale
2. change system wide locale
3. add 4.2 repo and try to upgrade with engine-setup

Actual results:
RuntimeError: Command '/opt/rh/rh-postgresql95/root/usr/bin/postgresql-setup' failed to execute

Expected results:
A successful upgrade or a clear error stating how to fix 

Additional info:

workaround:
check the expected system wide locale in /var/lib/pgsql/upgrade_rh-postgresql95-postgresql.log, temporary set it under /etc/locale.conf, execute engine-setup to upgrade to 4.2 and then you can safely revert back /etc/locale.conf to what you need