Enable source-level debugging with pydevd in Manila

To maximize developer productivity, it should be possible to do source-level debugging (i.e. runtime breakpoints, stepping, variable inspection) in Manila using pydevd in a Python IDE (i.e. Eclipse/PyDev or PyCharm). At least three other OpenStack projects (Keystone, Glance, Nova) were modified to support debugging, and those changes may be ported with a fair level of consistency to Manila (and potentially Cinder, etc.). The changes are limited to addition of CLI options to specify debugger connections, prevention of thread library greening by eventlet if debugging is active, and insertion of the debugger connection code in one place that works for all manila processes.