Handle install command

Install LTTng tools and modules, with few related commands:

* lttng-cluster install tools
* lttng-cluster install addons
* lttng-cluster install reload
* lttng-cluster install check
* lttng-cluster install reboot

The option -H/--hosts specifies the hosts to setup.

Signed-off-by: Francis Giraldeau <francis.giraldeau@gmail.com>
Fix hosts and user arguments

Default to current username if no username is specified. Also, make host
mandatory for now.

Signed-off-by: Francis Giraldeau <francis.giraldeau@gmail.com>