extmgr: use jboss module service load method

this is leftover from legacy code, now that I look at the jboss module
api there is no reason why not to get this via the module.

Change-Id: I5e0f7391dd34e17be1cc321f3114fac815093546
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>