Display a warning when an invalid host is specified via '-x'

[mattsfuller]

On a four node cluster with master, slave1, slave2 and slave3, when I run a presto-admin command say connector add hive and specify an invalid host name via -x, no warning is displayed about the host being invalid.
$./prestoadmin/presto-admin connector add hive -x blah
Deploying hive.properties connector configurations on: master
Deploying hive.properties connector configurations on: slave1
Deploying hive.properties connector configurations on: slave2
Deploying hive.properties connector configurations on: slave3
I suspect that this behavior is true for all presto-admin commands that support -x option, though I haven't tried it for all the commands.
Also opened the PR: #159
You may want to add a negative test for presto admin commands for this scenario.