Updating the paramiko version to 2.7.0 or greater

We have recently observed that ssh-keygen by default
create OPENSSH format for keys and paramiko got the
support for decrypting it from paramiko 2.7.0[1]

We have recenlty seen failures in whitebox plugin but
considering this can also be observe in tempest as well
it's good to update the version here too.

[1]: http://www.paramiko.org/changelog.html

Change-Id: I3603bbc9787e99fe864a4ec63d49fea6b90a45a8

requirements.txt

@@ -5,7 +5,7 @@ pbr!=2.1.0,>=2.0.0 # Apache-2.0
 cliff!=2.9.0,>=2.8.0 # Apache-2.0
 jsonschema>=3.2.0 # MIT
 testtools>=2.2.0 # MIT
-paramiko>=2.0.0 # LGPLv2.1+
+paramiko>=2.7.0 # LGPLv2.1+
 netaddr>=0.7.18 # BSD
 oslo.concurrency>=3.26.0 # Apache-2.0
 oslo.config>=5.2.0 # Apache-2.0