Telemetry: Don't use the full path to rdmsr on Linux.

This fixes the bots, even though when I log into the bots and run rdmsr
with the full path through Python, it works fine. This should be ok,
since Ubuntu includes the sbin directories in the path by default.

BUG=336556

Review URL: https://codereview.chromium.org/640323004

Cr-Commit-Position: refs/heads/master@{#299019}

tools/telemetry/telemetry/core/platform/linux_platform_backend.py

@@ -104,7 +104,7 @@ class LinuxPlatformBackend(
     return self._power_monitor.StopMonitoringPower()
 
   def ReadMsr(self, msr_number, start=0, length=64):
-    cmd = ['/usr/sbin/rdmsr', '-d', str(msr_number)]
+    cmd = ['rdmsr', '-d', str(msr_number)]
     (out, err) = subprocess.Popen(cmd,
                                   stdout=subprocess.PIPE,
                                   stderr=subprocess.PIPE).communicate()