plugins: http_static. Migrate to use api string type.

plugins: http_static. Migrate to use api string type.

This is not my core competency.
Reviews/feedback/suggestions welcomed. ;)

Tested with:
  rv = self.vapi.http_static_enable(
             fifo_size = 1,
             cache_size_limit = 1000000,
             prealloc_fifos = 0,
             private_segment_size = 0,
             uri="tcp://0.0.0.0/80",
             www_root = "/var/tmp/run/vpp/html"
             )

  DBGvpp# show http static server
  www_root /var/tmp/run/vpp/html, cache size 0 bytes, limit 1000000 bytes, evictions 0

Change-Id: I0f660753317ceedab89da1b65701a24d6f7145de
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>