file based and zookeeper based custom rule implementation

To enable dynamic custom rule configuration for vttablet, this patch implements two sets of dynamic rule updating mechanisms:

ZkCustomRule: zookeeper based custom rule manager
FileCustomRule: file based custom rule manager

This patch also sets up an interface, the CustomRuleManager. Future custom rule implementations which conforms with CustomRuleManager interface can be easily plugged in.
Looking at how you've wired things, this interface looks unnecessary:
- This interface is not used outside this package. So, there's no value in formalizing this.
- GetRules is unused, except for tests.