Fix switchblade Initialize() multi-stack buildpack deletion

When multiple buildpacks share the same name across different stacks
(e.g. ruby_buildpack for cflinuxfs4 and cflinuxfs5), cf delete-buildpack
requires -s <stack> to disambiguate. Without it, the command fails with:
'Multiple buildpacks named ruby_buildpack found. Specify a stack name
by using a -s flag.'

Parse the stack field from the /v3/buildpacks API response and iterate
over all resources, deleting each with the appropriate -s flag. This
fixes the CF integration test failure in Initialize() when both
cflinuxfs4 and cflinuxfs5 stacks are deployed.

Author: tnikolova82