[docs] add autodoc_mock_imports

The docs job has recently begun failing with:

autodoc: failed to import module 'connector' from module
'os_brick.initiator'; the following exception was raised: No module
named 'cryptography'

We're not directly importing cryptography in os-brick, so address this
by adding 'cryptography' to the autodoc_mock_imports list.

Change-Id: I5128d717f7a31fb9042f690ed7c17a3191f1a99e

doc/source/conf.py

@@ -50,6 +50,9 @@ openstackdocs_pdf_link = True
 openstackdocs_bug_project = 'os-brick'
 openstackdocs_bug_tag = ''
 
+# -- Options for autodoc ------------------------------------------------------
+autodoc_mock_imports = ['cryptography']
+
 # -- Options for LaTeX output -------------------------------------------------
 
 # The paper size ('letter' or 'a4').