Add checkov linter for cloudformation

Co-Authored-By: jhandsel <64368631+jhandsel@users.noreply.github.com>

test/linter/test_cloudformation_checkov.vader

@@ -0,0 +1,15 @@
+Before:
+  call ale#assert#SetUpLinterTest('cloudformation', 'checkov')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The default command should be direct):
+  AssertLinter 'checkov',
+  \ ale#Escape('checkov') . ' -f %t -o json --quiet --framework cloudformation '
+
+Execute(It should be possible to override the default command):
+  let b:ale_cloudformation_checkov_executable = '/bin/other/checkov'
+  AssertLinter '/bin/other/checkov',
+  \ ale#Escape('/bin/other/checkov') . ' -f %t -o json --quiet --framework cloudformation '
+