bugfix: Support paths with user's home shortcut

bugfix: Support paths with user's home shortcut

VSPERF will explicitly expand any '~' to the absolute path to the user's
home directory before call of subsystem.Popen method. Only calls,
which process configuration values are modified.

Change-Id: Ibd9399ae84bf4698c86bf8eff97ca6ce785fb3fb
JIRA: VSPERF-45
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>
Reviewed-by: Brian Castelli <brian.castelli@spirent.com>
Reviewed-by: Al Morton <acmorton@att.com>