crypto: Add support for Paramiko 2.x
crypto: Add support for Paramiko 2.x

Only use PyCrypto/PyCryptodome work-around with Paramiko 1.x and use
straight-forward Paramiko interface with 2.x.

TODO: Revert this and PyCrypto/PyCryptodome work-around when Paramiko
is upgraded to 2.x (ie replace `generate_keys(bits)` call with
`paramiko.RSAKey.generate(bits)`).

Change If88beeb3983705621fe736995939ac20b2daf1f3 added a work-around
for the partially-PyCrypto-compatible PyCryptodome causing Paramiko,
which has a dependency on PyCrypto, to break.  This work-around
entails implementing Paramiko internals (ie how to generate a key) in
Nova in a way compatible with both PyCrypto and PyCryptodom.

This work-around is itself a source of failure with Paramiko 2 which
has replaced the PyCrypto requirement with the cryptography Python
package.  As Paramiko no longer depends on PyCrypto, Nova doesn't have
an explicit PyCrypto requirement, and there's no implicit dependency
on PyCrypto, when Nova tries to import PyCrypto it fails.  Even if
PyCrypto was installed, the work-around would still fail because the
Paramiko interface that Nova is using as part of the work-around
changed with the major version change (ie 1.x => 2.x).

Change-Id: I5d6543e690a3b4495476027fd8a4894ff8c42bf6
Related-Bug: #1483132

cloudbase/nova,Juniper/nova,jianghuaw/nova,sebrandon1/nova,rahulunair/nova,Juniper/nova,klmitch/nova,vmturbo/nova,klmitch/nova,sebrandon1/nova,rajalokan/nova,gooddata/openstack-nova,rahulunair/nova,rajalokan/nova,openstack/nova,Juniper/nova,rajalokan/nova,alaski/nova,sebrandon1/nova,gooddata/openstack-nova,mahak/nova,phenoxim/nova,jianghuaw/nova,jianghuaw/nova,mikalstill/nova,hanlind/nova,openstack/nova,mikalstill/nova,hanlind/nova,cloudbase/nova,bigswitch/nova,bigswitch/nova,klmitch/nova,hanlind/nova,vmturbo/nova,Juniper/nova,gooddata/openstack-nova,klmitch/nova,jianghuaw/nova,vmturbo/nova,mikalstill/nova,phenoxim/nova,alaski/nova,rajalokan/nova,mahak/nova,vmturbo/nova,mahak/nova,openstack/nova,gooddata/openstack-nova,cloudbase/nova,rahulunair/nova