nat: nat44-ei configuration improvements

nat: nat44-ei configuration improvements

nat44-ed core configuration improvements & fixes [0-5] adjusted
for nat44-ei plugin.

Improvements:
 * repeating code converted to functions
 * simplified functions used for pool address, static mapping
and interface configuration.

Clean up:
 * remove obsolete code and logic persisted after plugin
separation from old SNAT plugin.

Fixes:
 * [0] return correct API behavior changed in [5]

Type: improvement

[0] https://gerrit.fd.io/r/c/vpp/+/33622
[1] https://gerrit.fd.io/r/c/vpp/+/33431
[2] https://gerrit.fd.io/r/c/vpp/+/33337
[3] https://gerrit.fd.io/r/c/vpp/+/33249
[4] https://gerrit.fd.io/r/c/vpp/+/32796
[5] https://gerrit.fd.io/r/c/vpp/+/32951

Signed-off-by: Filip Varga <fivarga@cisco.com>
Change-Id: Ie197faa576cb49acb3d218f14e00cb7d13ad9342