Make username configurable in the rpm SPEC file to avoid conflicts with existing salt user

[upendar245]

Description of Issue/Enhancement

Currently, the SaltStack SPEC file hardcodes the username salt for the service user. This can cause conflicts for users who already have an existing salt user account used for other purposes. It would be beneficial to make the username configurable during installation or provide a mechanism to override the default user.

Steps to Reproduce

1. Install SaltStack RPM on a system that already has a salt user created for a different application or purpose.
2. Observe that the RPM preinstall script fails or overwrites the existing salt user settings.

Suggested Solution

1. Modify the SPEC file to accept an environment variable or a build parameter that allows specifying a custom username.
2. Update the preinstall, postinstall, and preuninstall scriptlets to use this configurable username.
3. Provide documentation on how to use the new configuration options.

This enhancement would provide much-needed flexibility and prevent potential conflicts with existing system users.

Additional Context

Making the username configurable is a standard practice to avoid conflicts in environments where usernames might overlap across different applications. Implementing this change will make SaltStack more adaptable to various deployment environments.

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