A 2-cpu instance gets launched with `cpu: 1` setting

[r4victor]

Specify cpu: 1 in a workflow, like so:

- name: python-version
    provider: bash
    python: 3.7
    commands:
      - python --version
    resources:
      cpu: 1

And run it remotely. dstack will launch an instance with 2 cpus despite the setting. If you have a limit on a number of cpus in your AWS account, then you'll get VcpuLimitExceeded error. This may prevent trying dstack on free-tier AWS accounts with cpu limit set to 1.

The problem arises because dstack runs instances with at least 2 cpus and treats resources as minimum requirements.

Expected behavior:

1. Support cpu: 1; or
2. Clarify that resources specify minimum requirements, not exact ones, and the resources allocated may exceed the specified.