Add OR operator to cps-path

  steps to be followed :

1. Antlr grammer file changes should be made to recognise OR
2. Implement the code logic in order to add OR functionality
3. Add test cases
4. Update documentation
5. Demo to team

Here following are the antlr changes which made to recognise the OR

So,before there is and condition which is applied with leaf condition ,similarly we have added or to the antlr

path :cps-path-parser/src/main/antlr4/org/onap/cps/cpspath/parser/antlr4/CpsPath.g4

listElementRef :  OB leafCondition ( ( KW_AND | KW_OR) leafCondition)* CB 

multipleLeafConditions : OB leafCondition (( KW_AND | KW_OR) leafCondition)* CB 

to-do:
Need to implement the logic for OR Operation

But there are few doubts regarding this to be discussed