Handling CI variable overwrite issues

Inspired from CLOUD-309 and it's conclusion: 

Conclusion or closure

**At this point, all the variables are just overwritten. However, at the soonest, we should do the following: 

1. Show all the variables that may be overwritten to the user and ask them if they want to overwrite it or not?
2. Create an API for writing (or overwriting) one CI variable to the Gitlab Project

Another approach could be:

1. Create variables with prefix or suffix, for example: 
   1. QTCLOUD_CONAN_USER_${GITLAB_USER_ID}
   2. QTCLOUD_CONAN_PASS_${GITLAB_USER_ID}

Then, in the .gitlab-ci.yml, we should refer the variables with the above notation. We have to find if this is actually possible or not? If yes, then this is the best method.