Improve Loki config management

[cyriltovena]

Is your feature request related to a problem? Please describe.
Currently Loki configs file are not superseded by command line flags except for limits but limit
cannot be partially provided and fulfilled by default.

Describe the solution you'd like
Viper from https://github.com/spf13/viper could allow us to have correct order of config precedence with extra features like sane defaults and config reload. This should be fine to integrate even if cortex uses golang flag, viper support also it.

Additional context
#519
#518
https://github.com/cortexproject/cortex/blob/master/pkg/util/validation/limits.go#L65-L73

[daixiang0]

Is there any plan about promtail config?
Now i set a watch in this PR to reload config, if change to viper, would be easy.

[CyrilPeponnet]

+1 logcli / promtail and loki should be consistent and use viper / cobra to handle args / env var and config file.

[daixiang0]

some disguss on slack

[sh0rez]

Implemented in #980