Support for setting namespace with -n flag when running velero commands

[Joeavaikath]

For admin commands, we should be able to specify a non-velero server namespace to store the CRDs?

[Joeavaikath]

What does this look like?

• When you run admin commands they run on the same namespace as the velero server dont' they? Assumption false probably
• nonadmin commands wouldn't have this capability since it's just their specific namespace

[Joeavaikath]

Tested this, seems like the velero server isn't looking at namespaces other than the config namespace so how/why would this work?
@kaovilai thoughts?

running velero create backup test01 -n nacuser01
when the oadp-operator/velero server is on openshift-adp
would not work, since the namespace isn't being looked at

is there another use case that I'm missing?

[Joeavaikath]

Added it anyway using PersistentFlags in #113