Update .gitignore: exclude .env files and pyenv .python-version (#200)

1. It is common to keep environment variables in either `.env` or
`.envrc` files in the project root folder. To avoid accidental secrets
leak, exclude `.env` files.
2. To support kickstart files validation and syntax highlighting
`pykickstart` python package can be used together with "Kickstart
Language Support" plugin for VScode. If python environment is managed
with `pyenv`, make sure to exclude .python-version which is always local
for user's environment.

.gitignore

@@ -17,3 +17,6 @@ ubuntu/packages/*.bin
 ubuntu/packages/*.tar.gz
 .vscode
 */http/*.ks
+.python-version
+.env
+.envrc