LBaaS: fix handling pending create/update members and health monitors

When agent requests loadbalancer logical config from server,
server returns only active pool members and health_monitors.
Need to make server return also members and monitors which are in pending states.

Also a small refactoring moving ACTIVE_PENDING set to common place

Change-Id: I8e10004f199f982b055da18ea7a0e5e4d11fa7fb
Closes-Bug: #1259965
Merge "LBaaS: fix handling pending create/update members and health monitors"