Move metadata agent entry to its own file

Move metadata agent entry to its own file

Break main() and configs out of agent logic. This makes metadata
consistent with the L3 and DHCP agents [1, 2].

Along with [3], this is done to better delineate what the agent module
inner boundaries are about. More to follow to finish off the cleanup.

[1] https://review.openstack.org/#/c/145979/
[2] https://review.openstack.org/#/c/146024/
[3] https://review.openstack.org/#/c/146151/

Change-Id: I2842f7c21db6f6dabdd2549f91a9001220454c22