add configuration options for bitwarden role

[d-rk]

What this PR does / why we need it:

• make bitwarden docker images configurable (my nas is running on arm64 so I need to be able to exchange images. I will post a PR with arm64 support for ansible-nas soon)
• make the subdomain configurable
• add some config options for the backup container

Fixes #430

[davestephens]

This PR has a few things mashed in together - the bug fixes and functionality I'd like to get merged, but also my comments as per #446 apply here to, which i'm not happy merging at this point in time.

If you can separate (per contribution guidelines :-)) we can get this one merged asap.

[d-rk]

@davestephens I separated it into two PRs and removed the configuration options for the docker images

[sailhobie]

bitwarden_allow_signups: false

needs quotes around "false"