Use onlyOwner modifier in selectSetGroup of FullyBackedSortitionPool

We use an modifier that is already defined to check if a function is
called by a pool owner. The change results in a revert message change
from `Only owner may select groups` to `Caller is not the owner`.

Author: nkuba

contracts/FullyBackedSortitionPool.sol

@@ -91,9 +91,9 @@ contract FullyBackedSortitionPool is AbstractSortitionPool {
     uint256 groupSize,
     bytes32 seed,
     uint256 bondValue
-  ) public returns (address[] memory) {
+  ) public onlyOwner() returns (address[] memory) {
     PoolParams memory params = initializeSelectionParams(bondValue);
-    require(msg.sender == params.owner, "Only owner may select groups");
+
     uint256 paramsPtr;
     // solium-disable-next-line security/no-inline-assembly
     assembly {

test/fullyBackedSortitionPoolTest.js

@@ -173,7 +173,7 @@ contract("FullyBackedSortitionPool", (accounts) => {
 
       await expectRevert(
         pool.selectSetGroup(3, seed, minimumBondableValue, {from: alice}),
-        "Only owner may select groups",
+        "Caller is not the owner",
       )
     })