Check for RHSM_DISPLAY before loading any modules.

If RHSM_DISPLAY is set to a display number, for example,
and offscreen vncserver, DISPLAY will be set to that
value for the tests, so any windows get displayed
there. Remove rhsm_display init from the individual
test modules.
Merge pull request #840 from candlepin/alikins/fix_rhsm_display

Check for RHSM_DISPLAY before loading any modules.