Kubeseal: Add support for setting the controller name via an environment variable

**Which component**:
kubeseal

**Is your feature request related to a problem? Please describe.**
I work in an environment where the decision was made to install sealed-secrets using non-default namespace, and controller name. Because of that, it's needed to always include the name of the controller on the command-line. It would be nice to have the possibility to set the controller name, just like the controller namespace using an environment variable.

**Describe the solution you'd like**
Add the environment variable `SEALED_SECRETS_CONTROLLER_NAME` which is consumed by kubeseal to override the default controller name.

**Describe alternatives you've considered**
An alternative is to make use of a shell alias to set the namespace, and controller name. However, it preferable to use the CLI directly, instead of via aliases, as these are not always available (one notorious example that I can think of is when you're invoking `watch ...`; although, not applicable to kubeseal, it highlights the limitation).

**Additional context**
_None_

Start in the kubeseal CLI by tracing how the existing controller namespace environment variable is read and applied. Add support for SEALED_SECRETS_CONTROLLER_NAME to override the default controller name, and verify that kubeseal uses it when the variable is set.

Written by the indexing model from the issue text.