Fix warning and conditionalize questions when install SlapOS node

graph TB
A(start) --> B(slapos_master_url)
B --> C(slapos_web_master_url)
C --> D{register to SlapOS master?}
D --> |no| E(keep default computer_name)
E --> I(End)
D --> |yes| F(input computer_name)
F --> G(input slapostoken)
G --> H(input interface_name)
H --> I