webadmin: fix displaying binary gluster hook

When resolving a conflict on a hook with differet content
types in different servers, the binary content is getting displayed.
This now fixed by adding content type check before trying
to fetch the content from backend.

Fixed typo in getGlusterHookEntity().

Change-Id: Ia189162dc339bab135979b4801c964112b24df18
Bug-Url: https://bugzilla.redhat.com/973091
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>