Add support for composite keys

Add support for composite keys

Composite keys help us store objects which are unique
for a given set of pre-existing objects.
Eg: Many profiles can exist for a definition and its
key will have a definition name as a part of the composite key.
P2: Use a predefined interface for keys instead of generic interfaceP{}
P3: Add check for empty strings in stringer interface
P5: Add appropriate keys in other packages.

Issue-ID: MULTICLOUD-531
Change-Id: I314b1fbd718489ae8a45f0f38915c08ca32f9f43
Signed-off-by: Kiran Kamineni <kiran.k.kamineni@intel.com>