This commit also makes possible to use json files as configuration
files via `viper` configuration library. And contains breaking changes
to configuration.

Because of viper's limitations of detecting new map keys from env
variables, a little procedure goes trough prefixed env variables and
override viper settings by the parsed value of them. If the
configuration file is in yaml format env variables can contain correct
yaml strings and they will be parsed correctly. This behaviour is same
for json files too.

…nto upstream

# Conflicts:
#	auth_server/authn/github_auth.go
#	auth_server/authn/ldap_auth.go
#	auth_server/authn/static_auth.go
#	auth_server/authz/acl_mongo.go
#	auth_server/main.go
#	auth_server/mgo_session/mgo_session.go
#	auth_server/server/config.go