Add option required=(True|False) to get_bin_path and update modules

Added required as optional argument to get_bin_path(). It defaults to
false.  Updated following modules to use required=True when calling
get_bin_path():  apt_repository, easy_install, group, pip,
supervisorctl, and user.
Also removed _find_supervisorctl() from supervisorctl module and updated
_is_running() to not need it.

library/user

@@ -35,15 +35,8 @@ if os.path.exists('/etc/master.passwd'):
 # invoke adduser in lieu of useradd, nor pw in lieu of usermod.
 # That is, this won't work on FreeBSD.
 
-def get_bin_path(module, arg):
-    bin = module.get_bin_path(arg)
-    if bin is None:
-        module.fail_json(msg="Cannot find %s" % arg)
-    else:
-        return bin
-
 def user_del(module, user, **kwargs):
-    cmd = [get_bin_path(module, 'userdel')]
+    cmd = [module.get_bin_path('userdel', True)]
     for key in kwargs:
         if key == 'force' and kwargs[key] == 'yes':
             cmd.append('-f')
@@ -56,7 +49,7 @@ def user_del(module, user, **kwargs):
     return (rc, out, err)
 
 def user_add(module, user, **kwargs):
-    cmd = [get_bin_path(module, 'useradd')]
+    cmd = [module.get_bin_path('useradd', True)]
     for key in kwargs:
         if key == 'uid' and kwargs[key] is not None:
             cmd.append('-u')
@@ -103,7 +96,7 @@ Without spwd, we would have to resort to reading /etc/shadow
 to get the encrypted string.  For now, punt on idempotent password changes.
 """
 def user_mod(module, user, **kwargs):
-    cmd = [get_bin_path(module, 'usermod')]
+    cmd = [module.get_bin_path('usermod', True)]
     info = user_info(user)
     for key in kwargs:
         if key == 'uid':