added initial implementation of content environments

Please review this PR for:
- naming conventions: we already have implementation for foreman environments, however this PR is for `Lifecyle Environments` under content menu. 
- I'm using file names like: `content_environment.py` and `test_content_environment.py` Please add a comment if you have any other suggestion for file names.
- Also, naming convention for locators. I'm using like `content_env.name`

What's pending in this:
- We should have support of org creation/selection with login. thoughts?
- Need to move locators for name/label/description etc under `common_locators`. and this requires changes in activation keys functions too.