Change ENUMS used in config yangs for Strings

ENUMs are not properly serialzied in config/netconf and it causes failure when reading/writing data using RESTCONF + Loopback connection

Change-Id: I8f9da4d009cdd3a432dd031ecc3a7cb551454d3d
Signed-off-by: Maros Marsalek <mmarsale@cisco.com>