actually check we can run scm command for roles (#43315)

* actually check we can run scm command for roles
* a better error message than file not found
* more narrow exception hanlding
* refactor common functions for more extended use and further 'basic.py' separation

lib/ansible/module_utils/common/process.py

@@ -0,0 +1,43 @@
+# Copyright (c) 2018, Ansible Project
+# Simplified BSD License (see licenses/simplified_bsd.txt or https://opensource.org/licenses/BSD-2-Clause)
+
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+import os
+
+from ansible.module_utils.common.file import is_executable
+
+
+def get_bin_path(arg, required=False, opt_dirs=None):
+    '''
+    find system executable in PATH.
+    Optional arguments:
+       - required:  if executable is not found and required is true it produces an Exception
+       - opt_dirs:  optional list of directories to search in addition to PATH
+    if found return full path; otherwise return None
+    '''
+    opt_dirs = [] if opt_dirs is None else opt_dirs
+
+    sbin_paths = ['/sbin', '/usr/sbin', '/usr/local/sbin']
+    paths = []
+    for d in opt_dirs:
+        if d is not None and os.path.exists(d):
+            paths.append(d)
+    paths += os.environ.get('PATH', '').split(os.pathsep)
+    bin_path = None
+    # mangle PATH to include /sbin dirs
+    for p in sbin_paths:
+        if p not in paths and os.path.exists(p):
+            paths.append(p)
+    for d in paths:
+        if not d:
+            continue
+        path = os.path.join(d, arg)
+        if os.path.exists(path) and not os.path.isdir(path) and is_executable(path):
+            bin_path = path
+            break
+    if required and bin_path is None:
+        raise ValueError('Failed to find required executable %s in paths: %s' % (arg, os.pathsep.join(paths)))
+
+    return bin_path