dmg: migrate changed path

[kenhys]

This is follow-up #489. it should be rebased with #475, #489.

Before:

• Use old path for compatibility.
  • /etc/td-agent/td-agent.conf
  • /var/log/td-agent/

After:

• Use /etc/fluent and /var/log/fluent by default.
• Keep /etc/td-agent, /var/log/td-agent as possible (only upgrading)
  • Moving content under /etc/td-agent to /etc/fluent/
  • Moving content under /var/log/td-agent to /var/log/fluent
  • Making symlink from /var/log/td-agent /var/log/fluent
  • Making symlink from /etc/td-agent to /etc/fluent

[kenhys]

NOTE: just