misc: vpp-api - add __repr__ to VPPApiClient.

misc: vpp-api - add __repr__ to VPPApiClient.

Show the constructor of the object instead of the memory location.

api client: <VPPApiClient apifiles=None, testmode=False,
async_thread=True, logger=<logging.Logger object at 0x7f35d1bd2590>,
read_timeout=1, use_socket=False,
server_address='/tmp/vpp-unittest-TestGbpApi-pOKbWg/api.sock'>

vs.

api client: <vpp_papi.vpp_papi.VPPApiClient object at 0x7fa0003df750>

Type: feature

Change-Id: I1460d76f6796776dd525c0844e9e426ec2d52398
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>