add support for checkov for linting terraform files (#4006)

* add support for checkov for terraform

* add tests for checkov handler

* add basic linter config tests for checkov

* update supported tools and languages lists

* simplify ale_linters#terraform#checkov#Handle

* ensure "-o json --quiet" is always set for checkov

* add documentation for checkov including config options

* fix tests after changing handling of default options for checkov

* add checkov to list of tools in doc/ale.txt

ale_linters/terraform/checkov.vim

@@ -0,0 +1,41 @@
+" Author: Thyme-87 <thyme-87@posteo.me>
+" Description: use checkov for providing warnings via ale
+
+call ale#Set('terraform_checkov_executable', 'checkov')
+call ale#Set('terraform_checkov_options', '')
+
+function! ale_linters#terraform#checkov#GetExecutable(buffer) abort
+    return ale#Var(a:buffer, 'terraform_checkov_executable')
+endfunction
+
+function! ale_linters#terraform#checkov#GetCommand(buffer) abort
+    return '%e ' . '-f %t -o json --quiet ' . ale#Var(a:buffer, 'terraform_checkov_options')
+endfunction
+
+function! ale_linters#terraform#checkov#Handle(buffer, lines) abort
+    let l:output = []
+
+    let l:results = get(get(ale#util#FuzzyJSONDecode(a:lines, {}), 'results', []), 'failed_checks', [])
+
+    for l:violation in l:results
+        call add(l:output, {
+        \   'filename': l:violation['file_path'],
+        \   'lnum': l:violation['file_line_range'][0],
+        \   'end_lnum': l:violation['file_line_range'][1],
+        \   'text': l:violation['check_name'] . ' [' . l:violation['check_id'] . ']',
+        \   'detail': l:violation['check_id'] . ': ' . l:violation['check_name'] . "\n" .
+        \             'For more information, see: '. l:violation['guideline'],
+        \   'type': 'W',
+        \   })
+    endfor
+
+    return l:output
+endfunction
+
+call ale#linter#Define('terraform', {
+\   'name': 'checkov',
+\   'output_stream': 'stdout',
+\   'executable': function('ale_linters#terraform#checkov#GetExecutable'),
+\   'command': function('ale_linters#terraform#checkov#GetCommand'),
+\   'callback': 'ale_linters#terraform#checkov#Handle',
+\})