Fix misleading `uffizzi install` message related to namespaces

[gadkins]

After running uffizzi install HOSTNAME --namespace NAMESPACE the error message says “This will install in the default namespace” even if they already specified a namespace.

• Update the existing message to use the name of the namespace given by the user or use 'default' if no namespace is provided.
• Do not include this line: `To install to a different place, use options '--namespace' and/or '--context'/

Update the message as follows:

$ uffizzi install controller.example.com --email jdoe@example.com --namespace foobar

This command will install Uffizzi in the 'foobar' namespace of 'gke_my-project_us_east5_my-cluster-1' context. 

After installation, new environments created for account 'gadkins' will be deployed to this host cluster.

Okay to proceed? (Y/n)