JENKINS-25209: Add "reboot slave after build" for a slave template

Adds slave template option to reboot a slave after a build
Once reboot commences the slave is monitored, reconnects are started and
the slave is reenabled once reconnect succeeds.
Fixes NPEs in tag handling
Make constructors more tolerant of null-strings.
Fix EC2ComputerLauncher switch state handling and breaks-to-label