Fixed Enum processing in BaseType

There is an incorrect assumption that all Enum types will be a Set/Array.
Thanks to the minimal IT written for hardware_vtep, this assumption is caught for an enum type for "encapsulation_type" field :
"type": { "key": { "enum": "vxlan_over_ipv4", "type": "string" } }

The incorrect assumption caused null pointer exception on the IT run.
Caused by: java.lang.NullPointerException: null
	at org.opendaylight.ovsdb.lib.schema.BaseType$StringBaseType.populateEnum(BaseType.java:307)

This fix addresses that problem.

Change-Id: I1cf1a278b6963765cf90268b616de7d721d2ef30
Signed-off-by: Madhu Venugopal <mavenugo@gmail.com>