new changes to yang model

[hrafiee]

I added DNS based PKI model as an option to external-certificates also changed a few parts of document.

[rkrejci]

cases localPKI and DNSbasedPKI are inserted into a wrong place, please use pyang to validate your data model:

$ pyang -p /path-to-imported-models/ of-config-sara.yang 
of-config-sara.yang:1: warning: unexpected modulename "of-config" in of-config-sara.yang, should be of-config-sara
of-config-sara.yang:1064: error: unexpected keyword "case"
of-config-sara.yang:1075: error: unexpected keyword "case"

[hrafiee]

I solved the error syntax. I hope now it is no problem