Fix issue in api-ref for API "os-extra_specs"

1)Fix API for updating the value of the specified extra-spec key.
Change "POST" to "PUT" and use os-extra_specs/{key_id} instead of
flavors/{flavor_id}/os-extra_specs URI

2)Add flavor extra_spec delete API

Change-Id: I6acf9fd0e7a22ee3069f816da18abaa8f1f783cc
Merge "Fix issue in api-ref for API "os-extra_specs""