Add validation for namespace in nonkube site create

[lynnemorrison]

When creating a site verify the namespace configured follows proper format.

fixes #2207

[fgiorgetti]

The validation itself looks ok to me, but it has to be applied to all create, delete and update commands for all resources, in order to close #2207. As is, if someone tries to create a listener or any other resource using an invalid namespace, it is still allowed.

[fgiorgetti]

Validation seems to work. The validation is still missing in the token redeem command.

[lynnemorrison]

Validation seems to work. The validation is still missing in the token redeem command.

Sorry about that. I didn't realize tokens were used in nonkube, I only looked at the token issue command and saw it wasn't implemented and didn't look at the redeem command.