Fixes LP 1022961
Previously an attempt to set the root password would fail silently if
for example the driver did not support this action.
To avoid the user being misled that the password change had succeeded,
we now report any failure by changing the MEP for set_admin_password
RPC from cast to call.
Change-Id: I1fd7b925e9226d30892f3a7cc4ddb938ff678a55