lib:fix clear route-map cmd using DEFPY

lib:fix clear route-map cmd using DEFPY

Due to the wrong input argv id, "argv[idx_word]->arg"
fetched in-correctly and it clears all the route-maps instead of
specific one.

Now correct argv id is passed to clear the given route-map counters.

Also, use RMAP_NAME which allows to show list of configured
route-maps in the system.

After Fix:-

Ticket:#3407773
Issue:3407773

Testing: UT done

Before:
TORC11# clear route-map counters
  <cr>
  WORD  route-map name

After:

TORC11# clear route-map counters
  <cr>
  RMAP_NAME  route-map name
     my-as

Signed-off-by: Chirag Shah <chirag@nvidia.com>
Signed-off-by: Sindhu Parvathi Gopinathan's <sgopinathan@nvidia.com>