Fix #39 multi master support to minion

This will fix #39

And it will be backwards compatible with the current pillar configuration.

pillar.example

  
   # salt minion config:
   minion:
+
+    # single master setup
     master: salt
+    
+    # multi master setup
+    master:
+      - salt_master_1
+      - salt_master_2
+    
     fileserver_backend:
       - git
       - roots

salt/files/minion.d/_defaults.conf

 
 # Set the location of the salt master server. If the master server cannot be
 # resolved, then the minion will fail to start.
+# master:salt
+{%- if 'master' in cfg_minion -%}
+{%- if cfg_minion['master'] is not string %}
+master:
+  {% for name in cfg_minion['master'] -%}
+  - {{ name }}
+  {% endfor -%}
+{%- else %}
+{{ get_config('master', 'salt') }}
+{%- endif %}
+{% elif 'master' in cfg_salt -%}
+{%- if cfg_salt['master'] is not string %}
+master:
+  {% for name in cfg_salt['master'] -%}
+  - {{ name }}
+  {% endfor -%}
+{%- else %}
 {{ get_config('master', 'salt') }}
+{%- endif -%}
+{%- endif %}
 
 # If multiple masters are specified in the 'master' setting, the default behavior
 # is to always try to connect to them in the order they are listed. If random_master is