Add `environment` drop down to pod logs screen

In preparation for showing logs on the Operations tab, we will need to allow users to choose the a particular environment. We should expose an option to allow you to select the environment you would like to view, but default to production on initial load, if it exists.

• We should consider the implementation of how this is/was done for Metrics, since it shares the same need: https://gitlab.com/gitlab-org/gitlab-ce/issues/45738

Original Design

There is an "Environment" dropdown after the "Container" dropdown.

Updated Design

1. Introduce an environment dropdown
2. Change the order of the dropdowns to reflect the underlying data structure (Environment > Pod > Container)
3. Introduce labels above both the new and the existing dropdown to clarify what the dropdowns are