vppapigen map: raise ValueError when fieldname is python keyword

vppapigen map: raise ValueError when fieldname is python keyword

When working on the lb api, one of the field names was chosen as 'as'
(application server). Since 'as' is a python keyword, the field was
renamed to _1 in vpp_papi.

This changeset instead fails early with a descriptive message,
hopefully saving others time troubleshooting the issue.

    ValueError: Fieldname 'as' is a python keyword and
    is not accessible via the python API.

Type: feature
Change-Id: Ib048d97de0e392645540092e356cf8989848c947
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>