ParameterVariant constructor with hard coded string gets interpreted as bool

[tfoote]

rclcpp::parameter::ParameterVariant("bar", std::string("hello")) is intepreted as a string correctly.
rclcpp::parameter::ParameterVariant("bar", "hello") is interpreted as a bool with value true.

From testing in ros2/system_tests#14

[tfoote]

Probably need a const char * constructor explicity.