SSP-1966 Externalize default manage_integration_database config

Previously that (database) config entry's default value was
hard-coded in Liquibase changesets. Now the same default is in
ssp-config.default.properties:

  db_liquibase_manage_external_database_by_default=true

So in theory you can now prevent creation of all external
tables and views by overriding that property in your
SSP_CONFIGDIR/ssp-config.properties.

Because this config property only affects what Liquibase
writes into the database config table as the default value of
manage_integration_database and because Liquibase
then uses the latter to make all other decisions about
whether to run integrations changesets, modifying this
file system config property after initial startup won't do
much good. Mostly you'll just get a checksum error. It really
is all about what you want Liquibase to set up as the default
in the db.