Pilot executes `cassandra` directly, bypassing the container image entry point

[wallrj]

• Cassandra Pilot now executes cassandra directly, bypassing the Docker image entrypoint
• Added a configuration API with which to override configuration values in yaml and properties config files.
• This replaces the experimental search / replace code for seed_provider and snitch.
• This also the use of environment variables to configure the Cassandra node. Instead, those configuration options are now supplied as Pilot command line arguments and are written to the config files in a pre-start pilot hook.
• Added an integration test so that i can see this mechanism working, without having to wait for e2e tests.

Fixes: #346, #251

Release note:

NONE

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[jetstack-bot]

@wallrj: PR needs rebase.

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