CLI flags that are slices/arrays should not be plural

[cjellick]

For example, it currently is:

acorn install --cluster-domains

but we want

acorn install --cluster-domain

Related issue: https://github.com/acorn-io/acorn/issues/472. We fixed the docs to align with reality, but we actually want to change the cli

[cjellick]

only one i found was cluster-domain. The rest were already right

[cjellick]

verified