normalize -etcd_servers flag across all commands

The -etcd_servers flag is used inconsistently by the Kubernetes commands,
both externally and internally.

This patch fixes the issue by using the same type to represent a list of
etcd servers internally, and declares the -etcd_servers flag consistently
across all commands.

This patch should be 100% backwards compatible with no changes in behavior.
Merge pull request #536 from kelseyhightower/etcd_servers

normalize -etcd_servers flag across all commands