tidy string/primitive coercion

[ahgittin]

changes to use a Maybe allow for additional rules to do more coercion

[aledsage]

@ahgittin test failure looks related to changes (TypeCoercionsTest.testCoercePrimitiveFailures):

java.lang.AssertionError: Error from type coercion of maybe to boolean failed with wrong exception; expected match of Predicates.and(containsLiteral("String"),containsLiteral("boolean"),containsLiteral("maybe")) but got: org.apache.brooklyn.util.javalang.coerce.ClassCoercionException: Cannot coerce type String to java.lang.Boolean (maybe): adapting failed
	at org.apache.brooklyn.util.javalang.coerce.TypeCoercionsTest.assertCoercionFailsWithErrorMatching(TypeCoercionsTest.java:154)
	at org.apache.brooklyn.util.javalang.coerce.TypeCoercionsTest.testCoercePrimitiveFailures(TypeCoercionsTest.java:140)

[aledsage]

Other than failing unit test, LGTM. Happy for this to be merged once that is fixed @ahgittin

[ahgittin]

server-side testing looks broken or slow -- tests pass locally now. merging.