use m2crypto instead of rsa

The pubkey is converted from PKCS#1 to X.501 format to be usable by
M2Crypto.

Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org>

See: [upstream issue](https://bitbucket.org/sybren/python-rsa/issue/19/vulnerable-to-side-channel-attacks-on) and [relevant redhat bugzilla](https://bugzilla.redhat.com/show_bug.cgi?id=1170703)

@dmp42 PTAL
I think this script is bitrot anyhow - the proper way to do this is to import from core compat: https://github.com/docker/docker-registry/blob/9124375c560a974dacff6cb63634a74808327493/tests/test_mirrors.py#L12

... or to simply ignore this (old) script.