do not allow empty strings on FTP config, and allow to abort process

Hi Jeremy,

I modified the sFTP config code so it won't let empty strings in, and also give you the possibility to abort sFTP configuration at any of the steps. If config is aborted, then $FTP_HOST is set to null so the FTP file won't be created.

Also, changed 

    if [[ $FTP_HOST != "" ]]

to

    if [[ -n "$FTP_HOST" ]] 

just because it's shorter and a primary way to check if a var is null (just learnt it today, I must say).

Cheers!