better validation for memory field

[bhearsum]

Recently someone accidentally put a value that was meant for the distribution field into the memory field. This ended up with a memory field with a value of "mozilla119". This is obviously not a valid memory value.

We should only accept integers, or integer + operator in this field.

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1433466)

Work is ongoing for this in #958