Rails: Fix shared_namespace validation

MR: Fix validation bug for shared namespace (!190060 - merged)

Description

https://kubernetes.io/docs/concepts/overview/working-with-objects/namespaces/#namespaces-and-dns states that

all namespace names must be valid RFC 1123 DNS labels.

Which is defined at https://kubernetes.io/docs/concepts/overview/working-with-objects/names/#dns-label-names

Our validators for shared_namespace currently allows space and dot(.). We need to fix this before releasing the feature publicly.

Acceptance criteria

• Shared namespace should not allow . or space in its value.

Implementation plan

Update the validator for shared_namespace to remove these characters from the regex of valid characters.