keymanager: support key creation with gpg2

right now, it expects pinentry input:

[2015-11-02 23:27:35] INFO     - L#265  : leap.keymanager.openpgp:_gen_key - About to generate keys... This might take SOME time.
Exception in thread Thread-26:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 763, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/lib/python2.7/dist-packages/gnupg/_meta.py", line 626, in _read_response
    result._handle_status(keyword, value)
  File "/usr/lib/python2.7/dist-packages/gnupg/_parsers.py", line 866, in _handle_status
    raise ValueError("Unknown status message: %r" % key)
ValueError: Unknown status message: u'PINENTRY_LAUNCHED'

(from redmine: created on 2015-11-03, precedes #7563 (closed))