Provide snapshot-topologies support in orchestrator-client (and via api)

[sjmudd]

orchestrator has a --snapshot-topologies option not available via orchestrator-client.

This patch provides:

• support in orchestrator-client to support --snapshot-topologies
• support in the api interface to handle a /api/snapshot-topologies call which will do the same thing.

Output of orchestrator-client looks like:

$ orchestrator-client -c snapshot-topologies
Took 2.125752ms

Output of a call to the orchestrator api (e.g. https://orchestrator.example.com/api/snapshot-topologies`) shows:

{"Code":"OK","Message":"Topology Snapshot completed","Details":"Took 2.058672ms"}

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• code is formatted via gofmt (please avoid goimports) # note: using go 1.12 formatting
• code is built via ./build.sh

[shlomi-noach]

snapshot-topologies was written a long time ago and I haven't been practicing it in a while. The idea was to have some "git history" for the topologies; I think the feature needs to be revisited and get some serious testing.

[sjmudd]

Well Shlomi, you set it up here. It's getting testing... but to be honest I'm not sure if the content it generates is being used. Were you wanting to get rid of it that would be fine I think.

Having said that the PR was because I'm moving all access to orchestrator to use the http interface so that I can switch the orchestrator clusters I talk to more easily (rather than use the config file). So moving to orchestrator-client means that any existing scripts etc collecting data which used the orchestrator binary need to be adapted. This was missing.

This patch has been running since I made the PR and seems to be working fine.