Add support for setting sandbox mode for system groovy build step scripts.

The current implementation of system groovy script command (systemGroovyScriptCommand) does not support sandbox attribute being set.
Further, the current implementation uses an old deprecated dataformat that forces the configuration to go through a deprecated constructor that forcefully sets the sandbox mode to false. This means that even if sandbox is set to true using custom XML via configure, that will be reverted due to the execution path through the deprecated constructor.

The implementation in the pull request uses the modern format in groovy-plugin 2.0, and script-security plugin 1.24, allowing sandbox mode to be set.