PT-2133 - added basic support for MongoDB to pt-stalk

[fipar]

This branch adds basic MongoDB support to pt-stalk, adding the --mongo option, which:

• Disables the MySQL collectors,
• Collects db.currentOp(), db.isMaster(), sh.status(), rs.status() and db.serverStatus()
• One of sh.status or rs.status will fail as they are mutually exclusive, but the collect_mongo function does not check this and runs both anyway
• Provides trg_mongo_default as the default mongo trigger (count of 'inprog' current operations) if --function is not specified

It includes no tests of these functions as it does not provide a mongodb sandbox, but it does not break any of the existing pt-stalk tests.

[dbmurphy]

This should be moved to the new PT in go tools