Do not run neutron-ns-metadata-proxy as root on dhcp agent

Do not run neutron-ns-metadata-proxy as root on dhcp agent

Currently neutron-ns-metadata-proxy runs with root permissions when
namespaces are enabled on the dhcp agent because root permissions are
required to "enter" in the namespace. But neutron-ns-metadata-proxy
permissions should be reduced as much as possible because it is
reachable from vms.

This change allows to change neutron-ns-metadata-proxy permissions
after its startup through the 2 new options metadata_proxy_user and
metadata_proxy_group which allow to define user/group running metadata
proxy after its initialization. Their default values are
neutron-dhcp-agent effective user and group.

This change delegates metadata proxy management to metadata driver
methods in order to reuse the work already done on l3 agent side.

Permissions drop is done after metadata proxy daemon writes its
pid in its pidfile (it could be disallowed after permissions drop) and
after metadata proxy daemon binds its privileged server port (80).

Using nobody as metadata_proxy_user/group (more secure) is currently
not supported because:

* nobody has not the permission to connect the metadata socket,
* nobody has not the permission to log to file because neutron uses
  WatchedFileHandler (which requires read/write permissions after
  permissions drop).
This limitation will be addressed in a daughter change.

DocImpact
Closes-Bug: #1187107
Change-Id: I53e97254d560e608101010f67bd2dcdec81fb6a2