Support instances_retrievable for osb-clients to check existence of a service instance

[gberche-orange]

Expected behavior

As an osb-client,

• in order to check whether a service instance exists associated with a given id
• in order to check whether a given id was already used
• I need the OSB instances_retrievable feature to be implemented

Observed behavior

As a workaround, OSB client can be given a read-only access to the osb-cmdb

Affected release

Reproduced on version x.y
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[gberche-orange]

duplicate of #9