Update maru to allow mixed case variable names

[zachariahmiller]

Is your feature request related to a problem? Please describe

Maru or at least the schema only allows upper case letters in the variable names. This ticket it to do the following:

1. validate if this is a real issue in maru or just a schema issue
2. if its a real issue modify maru to allow mixed case characters in variable names
3. update the schema to reflect that mixed case characters are allowd

Additional context

This shows a schema error:

variables:
  - name: myVariable
    default: "value"

This does not:

variables:
  - name: MY_VARIABLE
    default: "value"