Add a `--yes` flag to bypass interactive prompt for `vuls prepare`

[Code0x58]

This is needed to run vuls prepare in a non-interactive environment, e.g. if you run through the Docker image without using --interactive:

time="Nov 11 18:50:32" level=info msg="sudo ... OK" 
time="Nov 11 18:50:32" level=debug msg="SSHResult: servername: my-host, cmd: set -o pipefail; aptitude -h, exitstatus: 127, stdout: bash: aptitude: command not found\r\n, stderr: , err: %!s(<nil>)" 
time="Nov 11 18:50:32" level=info msg="Below servers are needed to install dependencies" 
time="Nov 11 18:50:32" level=info msg="  - aptitude on my-host" 
time="Nov 11 18:50:32" level=info msg="Is this ok to install dependencies on the servers? [y/N]" 
time="Nov 11 18:50:32" level=error msg="Failed to prepare: EOF"

This comes around from scan/serverapi.go; in this code the "Below servers are needed to install dependencies" message would be better as something like "The following servers require dependencies to be installed:".