Zuul gate to check if the cli doc is generated

The below code adds check zuul gate to verify
that for every cli added to airshipctl appropriate
documentation is generated.

Closes: #280
Change-Id: Iba0f4393b9785190536aae2a13109ddb75fd0ef8

tools/git_diff_check

@@ -23,7 +23,7 @@ then
     exit 1
 fi
 
-git diff-files --quiet --ignore-submodules **/*.md
+git diff-files --quiet --ignore-submodules **/*.rst
 if [ $? -ne 0 ]
 then
     echo "git diff found modified cli help, please run make cli-docs"
@@ -38,7 +38,7 @@ then
     exit 1
 fi
 
-git diff-index --cached --quiet --ignore-submodules HEAD -- **/*.md
+git diff-index --cached --quiet --ignore-submodules HEAD -- **/*.rst
 if [ $? -ne 0 ]
 then
     echo "git diff found modified cli help, please run make cli-docs"