Improve openssl dependency handling

[konradkonrad]

Context is on gitter: https://gitter.im/ethereum/pyethapp?at=56e7bb0f3194fbd110971f04

Hi Everyone, So the segmentation fault that I'm experiencing described above is due to openssl library that comes with centos 7 . The command openssl ecparam -list_curves
secp384r1 : NIST/SECG curve over a 384 bit prime field
secp521r1 : NIST/SECG curve over a 521 bit prime field
prime256v1: X9.62/SECG curve over a 256 bit prime field
now pydev2p2 in crypto.py sets the curve = 'secp256k1' so when ecc.py (from pyelliptic) calls self.curve = OpenSSL.get_curve(curve)
you get 714 (I think this is misleading because the curve is not implemented in the library or perhaps disabled) and the function raw_check_key which calls key = OpenSSL.EC_KEY_new_by_curve_name(curve) returns a 'None' which is the equivalent C to NULL since openssl is a wrapper to the c-shared library. The check that is done to raise exception is if ( key==0) which is not the same as (key == None) . My suggestion would be to actually test whether openssl that include the curves required
during the init of pyethapp