Add flag '-L/--hosts.list' for outputing a list of the target hosts for subcommand 'command/script/push'

[windvalley]

For example:

$ gossh script foo66.shenzhen.bar.com "foo[01-20].[beijing,wuhan].bar.com" -L

Output:

foo66.shenzhen.bar.com
foo01.beijing.bar.com
foo01.wuhan.bar.com
foo02.beijing.bar.com
foo02.wuhan.bar.com
foo03.beijing.bar.com
foo03.wuhan.bar.com
foo04.beijing.bar.com
foo04.wuhan.bar.com
foo05.beijing.bar.com
foo05.wuhan.bar.com
foo06.beijing.bar.com
foo06.wuhan.bar.com
foo07.beijing.bar.com
foo07.wuhan.bar.com
foo08.beijing.bar.com
foo08.wuhan.bar.com
foo09.beijing.bar.com
foo09.wuhan.bar.com
foo10.beijing.bar.com
foo10.wuhan.bar.com

hosts (21) 

$ gossh script foo66.shenzhen.bar.com "foo[01-20].[beijing,wuhan].bar.com" -L | head 

Output:

hosts (21)
foo66.shenzhen.bar.com
foo01.beijing.bar.com
foo01.wuhan.bar.com
foo02.beijing.bar.com
foo02.wuhan.bar.com
foo03.beijing.bar.com
foo03.wuhan.bar.com
foo04.beijing.bar.com
foo04.wuhan.bar.com
foo05.beijing.bar.com    

$ gossh script foo66.shenzhen.bar.com "foo[01-10].[beijing,wuhan].bar.com"

Output:

Error: need flag '-e/--execute' or '-L/--hosts.list'