bidirectional value: Fix inconsistency for bidir param

bidirectional value: Fix inconsistency for bidir param

Fixes bi-directional param and enforces string type in
testcase configuration. Modifies all instance where bi
directional value is used to string.

 * Changes documentation to specify value type as needed
 * Changes traffic defaults to use string for bidir value
 * Fixes testcases configuration files to use bidir as str
 * Change testcase conf documentation to note enforcement
   of string type
 * Sets bidir to title format to keep consitency in traffic
   profile when passed to traffic generators
 * Fix pvp/pvvp check for bidir value
 * Raise exception if testcase configuration is not set
   as str for bidir value
 * Change Xena bi-dir check to string compare instead of
   typecasting to boolean to compare.

JIRA: VSPERF-282

Change-Id: I7c91fcd7333a1d1e1371bfd31700c01d7da268b5
Signed-off-by: Christian Trautman <ctrautma@redhat.com>