Requests with stream=True MUST be closed or else the connection will
never be returned to the connection pool. Both ContainerApiMixin.attach
and ExecApiMixin.exec_start were leaking in the stream=False case.
exec_start was modified to follow attach for the stream=True case as
that allows the caller to close the stream when done (untested).

Tested with:

    # Test exec_run (stream=False) - observe one less leak
    make integration-test-py3 file=models_containers_test.py' -k test_exec_run_success -vs -W error::ResourceWarning'
    # Test exec_start (stream=True, fully reads from CancellableStream)
    make integration-test-py3 file=api_exec_test.py' -k test_execute_command -vs -W error::ResourceWarning'

After this change, one resource leak is removed, the remaining resource
leaks occur because none of the tests call client.close().

Fixes docker#1293
(Regression from docker#1130)

Signed-off-by: Peter Wu <pwu@cloudflare.com>