Starting to add additional unit tests for VariableManager

Required some rewiring in inventory code to make sure we're using
the DataLoader class for some data file operations, which makes mocking
them much easier.

Also identified two corner cases not currently handled by the code, related
to inventory variable sources and which one "wins". Also noticed we weren't
properly merging variables from multiple group/host_var file locations
(inventory directory vs. playbook directory locations) so fixed as well.

lib/ansible/cli/__init__.py

@@ -34,7 +34,6 @@ from ansible import constants as C
 from ansible.errors import AnsibleError, AnsibleOptionsError
 from ansible.utils.unicode import to_bytes
 from ansible.utils.display import Display
-from ansible.utils.path import is_executable
 
 class SortedOptParser(optparse.OptionParser):
     '''Optparser which sorts the options by opt before outputting --help'''
@@ -479,7 +478,7 @@ class CLI(object):
         return t
 
     @staticmethod
-    def read_vault_password_file(vault_password_file):
+    def read_vault_password_file(vault_password_file, loader):
         """
         Read a vault password from a file or if executable, execute the script and
         retrieve password from STDOUT
@@ -489,7 +488,7 @@ class CLI(object):
         if not os.path.exists(this_path):
             raise AnsibleError("The vault password file %s was not found" % this_path)
 
-        if is_executable(this_path):
+        if loader.is_executable(this_path):
             try:
                 # STDERR not captured to make it easier for users to prompt for input in their scripts
                 p = subprocess.Popen(this_path, stdout=subprocess.PIPE)