VistATestClient: Modify GTM command to use gtm_dist

Rather than assuming that GTM commands are in system path, query
$gtm_dist environmental variable to find the path instead.

OSEHRA-Id: http://issues.osehra.org/browse/VAF-139
Change-Id: I2c00f0dd2b61bb85555e700cdfc3a63919fc8c66
Merge "VistATestClient: Modify GTM command to use gtm_dist"