Add update command and refactor atlas-cli to support be more extensible for adding new commands

[burov]

Add update command This PR introduce new command that can update application state and add latest features from atlas-app-toolkit

Now we implement this behavior only with Makefiles Now we have three separated makefiles: Makefile.vars, Makefile.common, Makefile

Makefile.vars contain variables which we use in Makefile.common too customize behavior

Makefile.common contain common commands which have almost all application which use atlas-app-toolkit This file will be regenerated when you call atlas update-app

Makefile include both files Makefile.vars and Makaefile.common, if we have custom command or need to override commands from Makefile.common we store this commands in Makefile

[Evgeniy-L]

@razamiDev, there was a mail thread (subject="Atlas CLI update command"). Per this email we figured out key points that can be effected by atlas update command:

• Extensible Makefile
• Extensible CLI flags set
• Extensible middleware chain

This PR covers the first point (Extensible Makefile). E.g. it allows to have a common Makefile part that can be updated during update command and allows to separate customized part and app specific part. Changes in main.go or middlewares chain is out of scope for this PR and should be addressed separately.