Bump up minimum cryptography version

...not because we strictly *need* newer cryptography, but rather because
distro packages have moved forward to the point where the 1.x series
won't compile from source and PyPI doesn't have wheels for them.

See changes like:

- https://github.com/pyca/cryptography/commit/6e7ea2e
- https://github.com/pyca/cryptography/commit/f88aea5

Change-Id: I1ff5b61873cf382c7a89873ed4ba6153f299262a

lower-constraints.txt

@@ -12,7 +12,7 @@ chardet==3.0.4
 cliff==2.11.0
 cmd2==0.8.1
 coverage==3.6
-cryptography==1.8.2
+cryptography==2.0.2
 debtcollector==1.19.0
 dnspython==1.14.0
 docutils==0.11

requirements.txt

@@ -12,5 +12,5 @@ requests>=2.14.2                        # Apache-2.0
 six>=1.9.0
 xattr>=0.4;sys_platform!='win32'        # MIT
 PyECLib>=1.3.1                          # BSD
-cryptography!=2.0,>=1.8.2               # BSD/Apache-2.0
+cryptography>=2.0.2                     # BSD/Apache-2.0
 ipaddress>=1.0.16;python_version<'3.3'  # PSF