Require paramiko >= 1.8.0

There is a well-known bug in the Paramiko SSH library that causes
these Exceptions:
SSHException: Error connecting via ssh: PID check failed. RNG must be
re-initialized after fork(). Hint: Try Random.atfork()

This bug has been fixed in Paramiko, and therefore we should require
the newer version for Cinder drivers that use Paramiko to function
properly.

Change-Id: I0844db1011f8b5c0e771b0e99a83027a13b6ea75
Fixes: bug 1150720
Merge "Require paramiko >= 1.8.0"