Call on dhcp-agent DhcpLocalProcess.restart() breaks dhcp

Call on dhcp-agent DhcpLocalProcess.restart() breaks dhcp

The bugfix implemented on Ib37651f7f802debd472ab292b148c2a2496063a3
creates the network_conf_dir only during __init__, and restart()
method calls disable(), then enable(), where disable will remove
the config directory and all configuration files.

Previous implementation re-created the network_conf_dir on the
interface_name setter (called from enable) with ensure_conf_dir=True.

Change-Id: I6afaea062c9eed5c63fc64fb0113f460b2cf35c5
Closes-Bug: 1410982