Horizon quota collection is suboptimal

Confirmed on:
stable/kilo
stable/juno

Issue:
When user accesses `horizon/project/access_and_security/` page, this triggers a call to api/network->tenant_floating_ip_list, and this happens for each tab with preload = True (default) which has any related quotas in table_actions field of the related table.

Details:
For example for keypairs tab it is
table_actions = (CreateKeyPair, ImportKeyPair, DeleteKeyPairs,
                         KeypairsFilterAction,)
Here `CreateKeyPair` will trigger call to quotas.tenant_quota_usages(request) check which will in turn call tenant_floating_ip_list.

On default stable/kilo and stable/juno the call to floating_ip_list happens 2 times on this page.
If we set preload = False in https://github.com/openstack/horizon/blob/master/horizon/tabs/base.py#L257 then floating_ip_list is called once, but will also happen for Keypairs tab, and 2 times for FloaitngIPs tab (once for quotas and another time to actually get the IPs).

Current quota retrieval mechanism is suboptimal and should be cached to not duplicate calls for tabs or allow doing calls to get specific quota (for example, only keypair quota or only floatingip quota).

Current code assumes fast/cheap API response which may sometimes not be true - for huge clouds for example.