Fix installation script

[viliakov]

Changes to CLI installation script:

• Introducing the default location (/usr/local/bin/) to install cli to with respect to the docs https://docs.stackstate.com/cli/k8sts-cli-sts#macos https://docs.stackstate.com/cli/k8sts-cli-sts#linux
• Checking if the cli is available via $PATH. If not suggesting to extend PATH variable.