Sessions: add healthcheck type as an optional parameter to environments

[olevski]

Amalthea supports currently 3 type checks:

• http
• tcp
• none

The default in amalthea is tcp. So we can defautl to the same value but allow people to specify http which is more robust. The TCP healthcheck can be successful even when the http server is not fully ready.

We should not make this mandatory and allow users to create an environment without worrying about this - unless they know what they are doing.