gradle: Use new Workspace.setDriver method to set bnd driver

This is instead of directly setting the -bnd-driver workspace property.

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
gradle: Add gestalt support

The gradle plugin will set the batch gestalt. One can also set the
bnd_gestalt property of the root project, either through gradle.properties
in the workspace root or via the gradle -P command line option. The value
of the bnd_gestalt property is a comma separated list of gestalt names.

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>