Add shell script style

We should have a style guide for bash scripts.

As discussed in https://github.com/apache/incubator-teaclave/pull/609, environment variables and internal shell variables are capitalized. All other variable names should be lower case. Some shell scripts in the project don't follow this convection.

Here is an example:
https://google.github.io/styleguide/shellguide.html

In addition, we can add a shell scripts format checker to the build pipeline.