`tahoe invite` fails without all three of the `--shares...` parameters

[exarkun]

$ tahoe -d ~/Scratch/tahoe-lafs/grid/introducer/ invite foo
Traceback (most recent call last):
  File "/home/exarkun/Work/python/tahoe-lafs/src/allmydata/scripts/runner.py", line 155, in dispatch
    d = defer.maybeDeferred(f, so)
  File "/home/exarkun/VirtualEnvs/tahoe-lafs/lib/python2.7/site-packages/twisted/internet/defer.py", line 151, in maybeDeferred
    result = f(*args, **kw)
  File "/home/exarkun/VirtualEnvs/tahoe-lafs/lib/python2.7/site-packages/twisted/internet/defer.py", line 1613, in unwindGenerator
    return _cancellableInlineCallbacks(gen)
  File "/home/exarkun/VirtualEnvs/tahoe-lafs/lib/python2.7/site-packages/twisted/internet/defer.py", line 1529, in _cancellableInlineCallbacks
    _inlineCallbacks(None, g, status)
--- <exception caught here> ---
  File "/home/exarkun/VirtualEnvs/tahoe-lafs/lib/python2.7/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
    result = g.send(result)
  File "/home/exarkun/Work/python/tahoe-lafs/src/allmydata/scripts/tahoe_invite.py", line 97, in invite
    "shares-needed": options["shares-needed"] or config.get('client', 'shares.needed'),
exceptions.AttributeError: '_Config' object has no attribute 'get'

This appears to be a regression included in 1.14.0.

[GitHub <noreply@…>]

In 7ce7cf7/trunk:

Merge pull request #1016 from LeastAuthority?/3650.tahoe-invite-regression

Fix regression in tahoe invite w/o shares parameters

Fixes: ticket:3650