Make secret input secret

[soapy1]

This (in part) resolves hashicorp/vagrant#12865. This enables the ui to be able to accept secret interactive input. For example, now when mounting SMB folders:

default: You will be asked for the username and password to usefor the SMB
default: folders shortly. Please use the proper username/password of your
default: account.
default:  
default: Username (user[@domain]): myusername
default: Password (will be hidden):   
  ==> default: Clearing any previously set network interfaces...

[chrisroberts]

👍