[Feature] Use the config file for configuration

[swapnilgm]

Feature (What you would like to be added):

• Introduce new flag --config-file which accepts the config file path.
• Use this json/yaml config file to configure etcd-backup-restore server.
• Also, centralise the validation for all flags

Motivation (Why is this needed?):
Currently there are too many flags for etcdbrctl server command. Configuring each flag on for command lengthens the command line drastically. Instead its better to have it in single config file with section for different correlated flags.

Approach/Hint to the implement solution (optional):

• Use library like https://github.com/ghodss/yaml or https://github.com/spf13/viper to parse the config file.

[amshuman-kr]

👍 for https://github.com/spf13/viper.