Shell: Created abstract class for cloud metadata authentication plugins

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Shell: Modify EC2Metadata auth plugin to use the abstract class

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Shell: Add GCE metadata auth plugin

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Shell: Updated crash.properties with the new available GCE auth plugin

Also, modified it to be more explicit about what the options are

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>