Gandalf and the council of White Wizards

[Haramu]

The Hobbit

"It appeared that Gandalf had been to a great council of the white wizards, master of lore and good magic... "

Who were these white wizards? Was Tolkien just referring to the White council?

Gandalf hinted the Dark Lord's Return
"...The North will be freed from that horror for many long years, I hope. Yet I wish he (Necromancer) were banished from the world!"...
Did Gandalf know Sauron would soon return?

[Inziladun]

Who were these white wizards? Was Tolkien just referring to the White council?

I would say so. The story was supposed to be the work of Bilbo, and at that point he likely had only a vague notion of what a "wizard" actually was. Oddly enough, it appears Elrond was not present, as Bilbo overhead that information about the "council" as Gandalf relayed it to Elrond.

Did Gandalf know Sauron would soon return?

He obviously had a foreboding that would occur, yes.

[Legate of Amon Lanc]

Inzil basically said it, I think.

Who were these white wizards? Was Tolkien just referring to the White council?
Definitely. In LotR and in the Unfinished Tales, Gandalf speaks about this in retrospect several times, and I think in the Tale of the Years in the appendices to LotR there is specifically a mention of the Council meeting that year. So we can safely assume that this is the same thing and we can look at it as just another, "simplified" way of mentioning this: through the eyes of Bilbo, who at the point of writing had really only very vague idea what such council would be about, or perhaps making it simplified for the audience (who he might have expected to be equally uneducated future generations of hobbits, for whom the "high" matters of the "great world" would be rather alien). That much from the in-world perspective :)

Did Gandalf know Sauron would soon return?

He was afraid that it won't be so easy. I think you can see that also from the way he talks to Frodo in the beginning of LotR. He always knew Sauron couldn't be so easily destroyed.