We've added the ability to override CLI defaults with environment files
[1] but then it got reversed while fixing a related bug [2]. We need to
put this ability back because otherwise we cannot customize
e.g. NeutronDhcpAgentsPerNetwork, or deploy with external Ceph with
custom FSID and key.
As a related change we have to remove hardcoded role counts from
overcloud-env.json, because they'd overwrite the counts passed via
CLI. (Furthermore, having the hardcoded default counts there doesn't
make too much sense now that we have custom roles, the counts should
already be properly defaulted in role_data.yaml.)
[1] Change ID I2722e118abde1cea6210148b16ac1ed3d9f235d8
[2] Change ID I80c663c23dfa27fa7b4ca2bf6b0101c132e5889b
Closes-Bug: #1632683
Change-Id: Iaf0adda44c84fcc552e743e4712d1dcf01daa7e4
71ebf2a31e