Requirements section should be turned to a map instead of list

Requirements section is currently a list, however since the get_attribute function allows to refer to a requirement by name, it forces the requirements names to be unique. This limitation implicitly forces the section to behave as a map, and I think this should be expressed explicitly by turning the section in to a map, like the current capabilities section.

current situation:

software:
type: tosca.nodes.SoftwareComponent
properties:...
attributes:
ip_address:

#which host???
requirements:

1. the get attribute expression is referring to host, however there can be multiple "host"
2. requirements, and each can have different attribute values.

• host: server_be
  relationship: tosca.relationships.HostedOn
• host: server_fe
  relationship: tosca.relationships.HostedOn

server_be:
type: tosca.nodes.Compute
...

server_fe:
type: tosca.nodes.Compute
...

proposed fix:

software:
type: tosca.nodes.SoftwareComponent
properties:...
attributes:
ip_address:

requirements:

1. in this case the reference will be mapped uniquely to the one matching requirement name.
   host_be: # names are forced to be unique
   target: server_be
   relationship: tosca.relationships.HostedOn
   host_fe: # names are forced to be unique
   target: server_fe
   relationship: tosca.relationships.HostedOn

server_be:
type: tosca.nodes.Compute
...

server_fe:
type: tosca.nodes.Compute
...