Description of problem:
When attempting to create a cache store, creation fails if the `class` property is left unset.

Version-Release number of selected component (if applicable):
JDG Plugin Pack Update-01 for JBoss ON 3.2.0
   Infinispan Plugin 6.0.1.Final-redhat-2

How reproducible:
Always

Steps to Reproduce:
1.  Install and start JON 3.2 system.
2.  Install JDG Plugin Pack Update-01 for JBoss ON 3.2.0.
3.  Install and start JDG server using clustered.xml configuration
4.  Import JDG 6.2 server into inventory.
5.  Configure JDG connection settings.
6.  Create a new Infinispan Cache:

    From JDG (127.0.0.1:9990) / infinispan / Cache Containers / clustered -> Create Child > Cache
    *   *New Resource Name*: `MyCacheA`
    *   *Kind of cache*: *distributed-cache*
    *   *Start*: *LAZY*
    *   *Batching*: *No*
    *   *Indexing*: *NONE*
    *   *Mode*: *SYNC*

7.  Create a new Cache Store:

    From JDG (127.0.0.1:9990) / infinispan / Cache Containers / clustered / Cache / MyCacheA -> Create Child > Store
    *   *New Resource Name*: `MyCacheStoreA`
    *   *Kind of store*:    *Generic Store*

Actual results:
Cache store creation fails and child history reports reason:

    JBAS014746: class may not be null, rolled-back=true

Expected results:
Cache store creation should be successful -OR- if a class is required and no default one is being provided by JDG, this property should be marked as required to prevent the create child wizard from completing until the user provides the required properties.

Additional info:
It is not clear to me whether a class is actually a required property or not. From the management plug-in it is not marked as required so my assumption from a user perspective is that it is not required. The error on the other hand leads me to believe that either the plug-in is actually submitting the value as "null" rather then "unset" or JDG isn't treating null as an undefined value.

In either case we need to to the right thing here. This also includes the other configuration options that are provided in this dialog. If for example one requires additional values while another does not then the wizard should only inform the user they are missing required properties when the selected configuration requires those additional values.