nat: nat44 enable/disable dynamic config

nat: nat44 enable/disable dynamic config

This patch changes initialization and configuration of NAT
plugin. Instead of allocating data structures at vpp plugin
initialization phase allocation and configuration happens
after calling enable API or CLI call. This reduces base VPP
memory footprint and also enables dynamic reconfiguration
of the NAT plugin.

Type: improvement
Change-Id: I42c069ee19a0311d043ac1f3f230d87bc8d2680f
Signed-off-by: Filip Varga <fivarga@cisco.com>