Move hardcoded labels to the i18n file

[mtfranchetto]

Description

With the introduction of i18n (with #175) we have the ability of internationalize PCluster Manager. However, the codebase is scattered with hardcoded labels that cannot be replaced at runtime with the language chosen by the user. We should find all the places where labels are hardcoded, and move them to the strings.json file.

Look at #175 for an example on how to do it.