Adding basic support for working with external postgres DB

[jackyalbo]

Explain the changes

1. Adding support to provide a secret reference to a postgres DB URL in noobaa CRD. The secret has to have a key called "db_url" and to be in this format: potgres://user:password@full DNS name of external server:port/db-name
2. Adding a CLI option to add postgres-url in the same format in 1. for example:

noobaa install --postgres-url="postgres://postgres:GWHMacB1ajy1SkY4Q5FH4myc60lJHTbF3I99LLCCwnLj32Je815cWd92JeQeBSiz@externalserver.namespace.svc:5432/nbcore"

CLI will create the expected secret and attach it to the noobaa system CRD

Issues: Fixed #xxx / Gap #xxx

Testing Instructions:

• Doc added/updated
• Tests added

[liranmauda]

@t-pirhonen @lallinger-arbeit you might find this helpful

[liranmauda]

LGTM