frontend: factor out code to add a single identifier

No functional change.
frontend: parse enums from input files
use identifiers parsed from input files
openflow_input: add ofp_nicira_controller_role enum
convert enum values to integers

Previously the enum values given to the backends were strings of C code.
Luckily the code was simple enough that it was also valid Python. Moving the
conversion to the frontend makes it easier to move enum definitions into the
input files, and cleans up some pyloxi code.
openflow_input: add all enums
remove canonical header file parsing

This has been completely replaced with the openflow_input files.
Merge branch 'parse-enums'