Consider unify tls files and addr arguments fetching

[artek-koltun]

We have these lines repeated in many places

tlsFiles, err := c.Flags().GetString(common.TLSFiles)
cobra.CheckErr(err)

addr, err := c.Flags().GetString(common.AddrCmdLineArg)
cobra.CheckErr(err)

We can consider if we can remove that duplication by:

• search in cobra if they already have a dedicated mechanism to solve such issue.
• extract into a separate func
• fetch into global vars and reuse them in cobra commands