Allow setting default Retention policy with Variable on Docker deployment

[yurividal]

Having a variable such as DEFAULT_RETENTION_POLICY to set the number of days in the default retention policy would make it a lot easier for deployments where storage is a concern.

[j-zarp]

I agree, that would be a very handy feature. BTW, the same request appears on issue #282

[agn-7]

Apparently, there is an env var for this purpose mentioned in hub.docker: DOCKER_INFLUXDB_INIT_RETENTION However, I don't know why it doesn't work! (i.e. issue #580)