fixes potential ext_pillar line mismatches

The previous matching method could lead to mismatches.
Let's assume the following variables :
<pre>
self.branch = "master"
self.rp_location = "http://github.com/xyz/project/pillar1"
</pre>
and the following configuration :
<pre>
ext_pillar:
  - git: master     http://github.com/xyz/project/pillar1
  - git: master http://github.com/xyz/project/pillar10
  - git: master http://github.com/xyz/project/pillar1
</pre>

The current code :
- would not match the first line, because there is more than 1 space
- would erroneously match the second line