DB settings can switch by "play.db.env" system properties.

This patch is can switch DB connection settings by system properties.
I want to change the target DB when phase of projects.
(devlopment, test, production ...)

It's looks like "Ruby on Rails" settings.

    development.db.default.driver="org.h2.Driver"
    development.db.default.url="jdbc:h2:mem:default"
    development.db.default.user="sa"
    development.db.default.password="sa"
    development.db.legacydb.driver="org.h2.Driver"
    development.db.legacydb.url="jdbc:h2:mem:legacy"
    development.db.legacydb.user="l"
    development.db.legacydb.password="g"
    production.db.default.driver="org.h2.Driver"
    production.db.default.url="jdbc:h2:mem:default_prod"
    production.db.default.user="sa_prod"
    production.db.default.password="sa_prod"
    production.db.legacydb.driver="org.h2.Driver"
    production.db.legacydb.url="jdbc:h2:mem:legacy_prod"
    production.db.legacydb.user="l_prod"
    production.db.legacydb.password="g_prod"

..and you have nothing to do if you don't want it.

    db.default.driver="org.h2.Driver"
    db.default.url="jdbc:h2:mem:default"
    db.default.user="sa"
    db.default.password="sa"
    db.legacydb.driver="org.h2.Driver"
    db.legacydb.url="jdbc:h2:mem:legacy"
    db.legacydb.user="l"
    db.legacydb.password="g"

Updating env variable occasionally breaks other test suites. You must restore it.