puppetboard/app: Reducing code redundancy for environment retreival and checking

Moving the envs variable out of the functions scope to the global scope,
this enables each function to reference and use these values.

Adding a new function check_env() to standardize the way to ensure that
the request environment exists, if it doesn't then abort with a 404.

This reduces 16 blocks of code and is in line with @daenney's suggestions

puppetboard/templates/index.html

@@ -42,7 +42,6 @@
   </div>
   <div class="four column row">
     <div class="column">
-      <span>Global Metrics:</span>
     </div>
     <div class="column">
       <h1 class="ui header darkblue no-margin-bottom">{{metrics['num_nodes']}}</h1>

puppetboard/templates/layout.html

@@ -40,17 +40,15 @@
       <a {% if endpoint == request.endpoint %} class="active item" {% else %} class="item" {% endif %}
         href="{{ url_for(endpoint, env=current_env) }}">{{ caption }}</a>
       {%- endfor %}
-      {% if envs|length > 0 %}
-        <div class="ui item dropdown">
-          Environments
-          <i class="dropdown icon"></i>
-          <div class="menu">
-            {% for env in envs %}
-              <a class="item {% if env == current_env %}active{% endif %}" href="{{url_for_environments(env)}}">{{env}}</a>
-            {% endfor %}
-          </div>
+      <div class="ui item dropdown">
+        Environments
+        <i class="dropdown icon"></i>
+        <div class="menu">
+          {% for env in envs %}
+            <a class="item {% if env == current_env %}active{% endif %}" href="{{url_for_environments(env)}}">{{env}}</a>
+          {% endfor %}
         </div>
-      {% endif %}
+      </div>
       <div class="item right"><a href="https://github.com/puppet-community/puppetboard" target="_blank">v0.1.0</a></div>
     </div>
     <div class="ui grid padding-bottom">