Unlocking Galactic Wolf-Rayet stars with $\textit{Gaia}$ DR2 II: Cluster
  and Association membership
  Galactic Wolf-Rayet (WR) star membership of star forming regions can be used
to constrain the formation environments of massive stars. Here, we utilise
$\textit{Gaia}$ DR2 parallaxes and proper motions to reconsider WR star
membership of clusters and associations in the Galactic disk, supplemented by
recent near-IR studies of young massive clusters. We find that only 18$-$36% of
553 WR stars external to the Galactic Centre region are located in clusters, OB
associations or obscured star-forming regions, such that at least 64% of the
known disk WR population are isolated, in contrast with only 13% of O stars
from the Galactic O star Catalogue. The fraction located in clusters, OB
associations or star-forming regions rises to 25$-$41% from a global census of
663 WR stars including the Galactic Centre region. We use simulations to
explore the formation processes of isolated WR stars. Neither runaways, nor low
mass clusters, are numerous enough to account for the low cluster membership
fraction. Rapid cluster dissolution is excluded as mass segregation ensures WR
stars remain in dense, well populated environments. Only low density
environments consistently produce WR stars that appeared to be isolated during
the WR phase. We therefore conclude that a significant fraction of WR
progenitors originate in low density association-like surroundings which expand
over time. We provide distance estimates to clusters and associations host to
WR stars, and estimate cluster ages from isochrone fitting.
