Update Pebble service name for admission-webhook

[mvlassis]

Closes #136.

This PR simply changes the name of the Pebble service in the admission-webhook rock to match the container name defined in the charm operator's code, found here. This way, only one Pebble service is created.

To test:

• Clone the repository and checkout to this branch
• cd to the admission-webhook directory and run tox -e pack
• tox -e export-to-docker
• Clone the admission-webhook-operator repository, found here.
• Change the value of oci-image in metadata.yaml to the image name that was exported (prepending docker.io/library/ to the image name).
• Run the integration tests with tox -e integration -- --model kubeflow
• Tests should pass :)

[DnPlas]

Really nice catch! Thanks @mvlassis !