Pig should exclude hadoop conf in local mode

Currently, the behavior for hadoop conf look up is:

• in local mode, if there is hadoop conf, bail out; if there is no hadoop conf, launch local mode
• in hadoop mode, if there is hadoop conf, use this conf to launch Pig; if no, still launch without warning, but many functionality will go wrong

We should bring it to a more intuitive way, which is:

• in local mode, always launch Pig in local mode
• in hadoop mode, if there is hadoop conf, use this conf to launch Pig; if no, bail out with a meaningful message