Add keyword needed by guidelines

Add keyword needed by guidelines

The new variable naming guidelines [1] specify the use of a
new keyword named Utils.Set_User_Configurable_Variable_Default
when the default value for an user configurable variable
needs to be computed at the suite setup, either using
BuiltIn.Evaluate or even invoking other, more complex
keywords (that look around in the environment or do other
stuff like that).

[1] https://wiki.opendaylight.org/view/Integration/Test/Test_Code_Guidelines#User_configurable_variables

Change-Id: I08982748380c54b483383fbcd07a1b6ce617d1fb
Signed-off-by: Jozef Behran <jbehran@cisco.com>