Set GARDEN_EXTERNAL_IP on cell in digitalocean provisioner.
Set GARDEN_EXTERNAL_IP on cell in digitalocean provisioner.

[#86461402]

Signed-off-by: David Julia <ed0cc118a79fb117c6af81e058eba4f0ecd53e11@pivotal.io>

davidwadden/lattice-release,davidwadden/lattice-release,datachand/lattice,Klaudit/lattice,datachand/lattice,mikegehard/lattice,cloudfoundry-incubator/ltc,davidwadden/lattice,davidwadden/lattice,SrinivasChilveri/lattice,svrc-pivotal/lattice,svrc-pivotal/lattice,datachand/lattice,Klaudit/lattice,felipeg48/lattice,mikegehard/lattice,datachand/lattice,felipeg48/lattice,Klaudit/lattice,svrc-pivotal/lattice,Klaudit/lattice,svrc-pivotal/lattice,SrinivasChilveri/lattice,SrinivasChilveri/lattice,cloudfoundry-incubator/ltc,SrinivasChilveri/lattice