tsh does not respect user field in profile

[russjones]

Problem

tsh does not respect the user field in ~/.tsh/profile.

Reproduction

1. Configure Teleport to use OIDC or SAML.
2. tsh --proxy=localhost login
3. tsh --proxy=localhost ssh localhost

During Step 3 you will be redirected to your identity provider to login again because tsh only reads profiles if no proxy is set: https://github.com/gravitational/teleport/blob/master/tool/tsh/common/tsh.go#L514-L519

Solution

• If --proxy is not set, read the ~/.tsh/profile symlink.
• If --proxy is set, read profile from ~/.tsh/<proxy-addr>.yaml.

For both cases, after reading the profile, override the values with command line flags.