enable --yes/-y flag for commands requiring confirmation

[hakamadare]

building unit tests has revealed that we could really use a --yes or --force flag to facilitate automated testing of the nepho UI.

test_nepho_cloudlet_install (tests.cloudlet_test.a_TestNephoCloudlet) ... ERROR
...
======================================================================
ERROR: test_nepho_cloudlet_install (tests.cloudlet_test.a_TestNephoCloudlet)
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Traceback (most recent call last):
  File "/Users/huit/python/nepho/nepho/tests/cloudlet_test.py", line 54, in test_nepho_cloudlet_install
    app.run()
  File "/Users/huit/.virtualenvs/nepho/lib/python2.7/site-packages/cement-2.1.4.dev20131029203905-py2.7.egg/cement/core/foundation.py", line 493, in run
    self.controller._dispatch()
  File "/Users/huit/.virtualenvs/nepho/lib/python2.7/site-packages/cement-2.1.4.dev20131029203905-py2.7.egg/cement/core/controller.py", line 448, in _dispatch
    func()
  File "/Users/huit/.virtualenvs/nepho/lib/python2.7/site-packages/cement-2.1.4.dev20131029203905-py2.7.egg/cement/core/controller.py", line 454, in _dispatch
    func()
  File "/Users/huit/python/nepho/nepho/nepho/cli/cloudlet.py", line 160, in install
    self.cloudletManager.new(name, selected_dir, url)
  File "/Users/huit/python/nepho/nepho/nepho/core/cloudlet.py", line 175, in new
    return Cloudlet(name, cloudlet_path, url)
  File "/Users/huit/python/nepho/nepho/nepho/core/cloudlet.py", line 28, in __init__
    self.clone(url)
  File "/Users/huit/python/nepho/nepho/nepho/core/cloudlet.py", line 89, in clone
    exit(1)
  File "/Users/huit/.virtualenvs/nepho/lib/python2.7/site.py", line 403, in __call__
    raise SystemExit(code)
SystemExit: 1
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Cloudlet install failed.
'git clone -v https://github.com/cloudlets/nepho-example.git /Users/huit/.nepho/cloudlets/nepho-example' returned exit status 128: fatal: destination path '/Users/huit/.nepho/cloudlets/nepho-example' already exists and is not an empty directory.


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