Allow to change application environment by ENV

[s3tezsky]

What is happening

When #2055 is merged, there should be easier to use ENV variable APP_ENV to change application environment. It will be important to find all ocurences and connected stuff (#1615 may help with implementation).

In current state there needs to be a file (DEVELOPMENT, TEST or PROD) and changing the environment in a different way is not possible.

Expected result

We want to have possibility to override it by passing APP_ENV into application. Fallback to current state shoul be kept (for case there is not the ENV set).