occm: add a new batch pools members update endpoint support

[kayrus]

What this PR does / why we need it:

This PR adds a support for a new potential octavia endpoint that allows to update pool members for multiple pools with one API call.

See https://bugs.launchpad.net/bugs/2125786 and sapcc/octavia#57

Which issue this PR fixes(if applicable):
fixes #2858

Special notes for reviewers:

Release note:

occm: add a new batch pools members update endpoint support

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