docs: Fix formatting of GCE_METADATA_HOST

[ijrsvt]

The existing docs only show GCE_METADATA_ROOT. This PR adds a space between them so that Sphinx will separate them into separate entries.

The new HTML:

<dl class="data">
<dt id="google.auth.environment_vars.GCE_METADATA_HOST">
<code class="sig-name descname">GCE_METADATA_HOST</code><em class="property"> = 'GCE_METADATA_HOST'</em><a class="headerlink" href="#google.auth.environment_vars.GCE_METADATA_HOST" title="Permalink to this definition">¶</a></dt>
<dd><p>port to be
used for GCE metadata requests.</p>
<p>This environment variable was originally named GCE_METADATA_ROOT. The system will
check this environemnt variable first; should there be no value present,
the system will fall back to the old variable.</p>
<dl class="field-list simple">
<dt class="field-odd">Type</dt>
<dd class="field-odd"><p>Environment variable providing an alternate hostname or host</p>
</dd>
</dl>
</dd></dl>

<dl class="data">
<dt id="google.auth.environment_vars.GCE_METADATA_ROOT">
<code class="sig-name descname">GCE_METADATA_ROOT</code><em class="property"> = 'GCE_METADATA_ROOT'</em><a class="headerlink" href="#google.auth.environment_vars.GCE_METADATA_ROOT" title="Permalink to this definition">¶</a></dt>
<dd><p>Old environment variable for for GCE_METADATA_HOST.</p>
</dd></dl>

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[ijrsvt]

@arithmetic1728 do you know why Kokoro is not running?

[arithmetic1728]

@ijrsvt I am not sure. Just triggered kokoro to re-run.

[ijrsvt]

@arithmetic1728 It looks like it passes now! Can you add the automerge label? Thanks :)