[BZ 958922] RFE: support for passing plugin config when creating child
resources

Now we support all 4 possible 'ways' to create a resource on RHQ

1. regular resource (was already implemented) - when REST client create
resources that agent would have discovered. Such resources is not possible
to create via UI or CLI

2. content-based resources (was already implemented)

3. resource children: similar to content-based resources, UI analogy is
"Create Child". Create child history exists. - possible resource duplicities
are handled on agent side.

4. manual resources: Resources that could be only manually Imported from UI,
create child history does not exist for those, and their resource key highly
depends on given pluginConfiguration (although resourceName can be passed).
Duplicates are handled on server side, since underlying API does not report
an error when manually imported resource already exists.

For 3. and 4. there's client can optinaly pass only required plugin/resource
configurations. In rest api code, we retrieve all the defaults from resource
type and then apply passed values on top.