Darwin: postgres_schema.present throwing exception

[noelmcloughlin]

On Darwin the POSIX user/group created by default is named "_postgres". An exception is thrown because salt tries to "runas: postgres" but this user does not exist.

OS: Darwin 15.6.0, Salt Version: saltstack: stable 2017.7.1 (bottled), HEAD

      ID: postgres_schema-uuid_ossp
Function: postgres_schema.present
    Name: uuid_ossp
  Result: False
 Comment: An exception occurred in this state: Traceback (most recent call last):
            File "/usr/local/Cellar/saltstack/2017.7.1/libexec/lib/python2.7/site-packages/salt/state.py", line 1837, in call
              **cdata['kwargs'])
            File "/usr/local/Cellar/saltstack/2017.7.1/libexec/lib/python2.7/site-packages/salt/loader.py", line 1794, in wrapper
              return f(*args, **kwargs)
            File "/usr/local/Cellar/saltstack/2017.7.1/libexec/lib/python2.7/site-packages/salt/states/postgres_schema.py", line 74, in present
              schema_attr = __salt__['postgres.schema_get'](dbname, name, **db_args)
            File "/usr/local/Cellar/saltstack/2017.7.1/libexec/lib/python2.7/site-packages/salt/modules/postgres.py", line 2108, in schema_get
              db_password=db_password)
            File "/usr/local/Cellar/saltstack/2017.7.1/libexec/lib/python2.7/site-packages/salt/modules/postgres.py", line 2160, in schema_list
              password=db_password)
            File "/usr/local/Cellar/saltstack/2017.7.1/libexec/lib/python2.7/site-packages/salt/modules/postgres.py", line 428, in psql_query
              password=password)
            File "/usr/local/Cellar/saltstack/2017.7.1/libexec/lib/python2.7/site-packages/salt/modules/postgres.py", line 367, in _psql_prepare_and_run
              rcmd, runas=runas, password=password, host=host, port=port, user=user)
            File "/usr/local/Cellar/saltstack/2017.7.1/libexec/lib/python2.7/site-packages/salt/modules/postgres.py", line 184, in _run_psql
              ret = __salt__['cmd.run_all'](cmd, python_shell=False, **kwargs)
            File "/usr/local/Cellar/saltstack/2017.7.1/libexec/lib/python2.7/site-packages/salt/modules/cmdmod.py", line 1699, in run_all
              **kwargs)
            File "/usr/local/Cellar/saltstack/2017.7.1/libexec/lib/python2.7/site-packages/salt/modules/cmdmod.py", line 409, in _run
              'User \'{0}\' is not available'.format(runas)
          CommandExecutionError: User 'postgres' is not available
 Started: 01:40:38.529985
Duration: 3.858 ms
 Changes: 

[noelmcloughlin]

Based on salt.grains.core.py (if grains['kernel'] == Darwin THEN grains['os'] = "MacOS"). So we could update osmap.yaml with 'MacOS' stanza if needed.

[noelmcloughlin]

Addressed by #164