Add support to set or unset extra_spec

Registered by xiexs

Add support to set or unset extra_spec for the OSC(openstackclient).
For instance:
usage: openstack flavor set <flavor> <key=value> [<key=value> ...]
             and
             openstack flavor unset <flavor> <key=value> [<key=value> ...]
Positional arguments
             <flavor> Name or ID of flavor
             <key=value> Extra_specs to set/unset

Blueprint information

Status:
Complete
Approver:
None
Priority:
Undefined
Drafter:
xiexs
Direction:
Needs approval
Assignee:
xiexs
Definition:
Obsolete
Series goal:
None
Implementation:
Unknown
Milestone target:
None
Completed by
Dean Troyer

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