gluster-nagios-monitoring: fixing extra login popup from trends tab.

    gluster-nagios-monitoring UI plugin is using legacy http://host:/api
URL for Rest API calls and thats the reason for extra additional popup
in the trends tab.

   Changing the URL to to use /ovirt-engine/api also sending the
"Prefer:persistent-auth"  header with all Rest API calls to keep
the wedamin's Rest API session active.

Change-Id: I4dba4af5cef01a5d30ed24831bd2333ba9ae05bd
Bug-Url: https://bugzilla.redhat.com/1143041
Signed-off-by: Ramesh Nachimuthu <rnachimu@redhat.com>