Make llama stack build not create a new conda by default

[ashwinb]

What does this PR do?

So far llama stack build has always created a separate conda environment for packaging the dependencies of a distribution. The main reason to do so is isolation -- distributions are composed of providers which can have a variety of potentially conflicting dependencies. That said, this has created significant annoyance for new users since it is not at all transparent. The fact that llama stack run is actually running the code in some other conda is very surprising.

This PR tries to make things better.

• Both llama stack build and llama stack run now accept an --image-name argument which represents the (conda, docker, virtualenv) image you want to operate upon.
• For the default (conda) mode, the script checks if a current conda environment exists. If one exists, it uses it.
• If --image-name is provided, that option is used. In this case, an environment is created if needed.
• There is no automatic llamastack- prefixing of the environment names done anymore.

Test Plan

Start in a conda environment, run llama stack build --template fireworks; verify that it successfully built into the current environment and stored the build file at $CONDA_PREFIX/llamastack-build.yaml. Run llama stack run fireworks which started correctly in the current environment.

Ran the same build command outside of conda. It failed asking for --image-name. Ran it with llama stack build --template fireworks --image-name foo. This successfully created a conda environment called foo and installed deps. Ran llama stack run fireworks outside conda which failed. Activated a different conda, ran again, it failed saying it did not find the llamastack-build.yaml file. Then used --image-name foo option and it ran successfully.

[raghotham]

loving the polish