SPARK-1099:Spark's local mode should probably respect spark.cores.max by default

This is for JIRA:https://spark-project.atlassian.net/browse/SPARK-1099
And this is what I do in this patch (also commented in the JIRA) @aarondav

 This is really a behavioral change, so I do this with greate caution, adn welcome any review advice:

1 I change the "MASTER=local" pattern of create LocalBackEnd . In the past, we passed 1 core to it . now it use a default cores
The reason here is that when someone use spark-shell to start local mode , Repl will use this "MASTER=local" pattern as default.
So if one also specify cores in the spark-shell command line, it will all go in here. So here pass 1 core is not suitalbe reponding to our change here.
2 In the LocalBackEnd , the "totalCores" variable are fetched following a different rule(in the past it just take in a userd passed cores, like 1 in "MASTER=local" pattern, 2 in "MASTER=local2" pattern"
rules: 
a The second argument of LocalBackEnd 's constructor indicating cores have a default value which is Int.MaxValue. If user didn't pass it , its first default value is Int.MaxValue
b In getMaxCores, we first compare the former value to Int.MaxValue. if it's not equal, we think that user has passed their desired value, so just use it 
c. If b is not satified, we then get cores from spark.cores.max, and we get real logical cores from Runtime. And if cores specified by spark.cores.max is bigger than logical cores, we use logical cores, otherwise we use spark.cores.max
3 In SparkContextSchedulerCreationSuite 's test("local") case, assertion is modified from 1 to logical cores, because "MASTER=local" pattern use default vaules.