[c++17] Corrected call of boost::make_optional()
[c++17] Corrected call of boost::make_optional()

Without specifying "boost" namespace C++17 compiler treats
make_optional() as std::make_optional() and produces error:
./../src/kudu/master/placement_policy-test.cc:1014:49: error:
no viable conversion
from 'optional<decay_t<std::__cxx11::basic_string<char> > >'
(aka 'optional<std::__cxx11::basic_string<char> >')
to 'const boost::optional<std::string>'
(aka 'const optional<basic_string<char> >')
        ASSERT_OK(policy.PlaceTabletReplicas(3,
        make_optional(string(label)), &result));

Change-Id: I553ce8275508914cfa322a05fe2adfcd802372ad
Reviewed-on: http://gerrit.cloudera.org:8080/15781
Tested-by: Kudu Jenkins
Reviewed-by: Bankim Bhavsar <3aac13853bde1b179487acd91435d1ded94913cb@cloudera.com>
Reviewed-by: Alexey Serbin <864f6b82bd94e8dda64141d4274519ee66e0102e@cloudera.com>

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