Add dependencies for paramiko 2.0

Paramiko version 2.0 has been released. It now uses the Python library
cryptography. Installing this requires additional system packages. This
commit adds in the appropriate packages required by cryptography based
on its documentation [1].

An alternative approach would have been to constrain the version of
Paramiko however the project describes the 1.x versions as relying on
insecure dependencies [2].

[1] https://cryptography.io/en/latest/installation/
[2] http://www.paramiko.org/installing.html

Change-Id: I337770a4c5655907367be07d97e9eb91eac8c890

other-requirements.txt

@@ -14,3 +14,7 @@
 # TODO(odyssey4me) remove this once https://review.openstack.org/288634 has merged
 # and the disk images are rebuilt and redeployed.
 curl
+
+# Requirements for Paramiko 2.0
+libssl-dev
+libffi-dev