Role based policy implementation

It adds role bases access control in all openstack apis of
MagnetoDB.The policy file lives in /etc/magnetodb/policy.json.

A rule "mdb:create_table":"role:admin" means that anyone having
role admin can create a table.

A rule "mdb:list_tables":"!" means that nobody would be allowed
to access the api applying this check.

A rule "mdb:list_tables":"" means that everyone is allowed to
access the api.

The rule "default" applies for the rules which are not present
in the policy.json file. For example if you remove all the rules
and keep just "default":"" then all the apis are accessible by
everyone.

bp : support-roles

Change-Id: I213aea56c47261d90a2ab0f8b5042bae9da3d8bc
Merge "Role based policy implementation"