Using Targets: Add setTarget for Rapid Management

[zvikazend]

When you run commands on specific target, you execute commands like:
zs-client.sh --target= <--params 1,2,3>
or
zs-client.sh <--params 1,2,3> --target=

To quickly work with direct command + params patterns on a single target at a time,
a command like 'setTarget ' can be used, which will set the default target for all consequent commands.

Notes:

1. Using --target in a command will override default / pre-set target.
2. If default target should be unset, just run 'zs-client setTarget' or 'zs-client setTarget=' to unset.
3. When a default target is set, zs-client will show the used target on every execution, with full target details, so there is no question where the command is going to be executed, i.e.:
   Using Target: 'My Cluster' => http://my-cluster.com:10081