Improvements to observium_installscript.sh for Agent deployments

Some issues identified with the observium_installscript.sh deployment for unix agents that I wanted to report to hopefully get fixed.    Since they are all in the same script I've aggregated them here opposed to creating a seperate ticket for each.

1) when running the observium_installscript.sh agent deployment script it assumes that /opt/observium/scripts exists on the system.   This is NOT the case when installing a system and even if so, may not have the correct/updated files.     The script should either pull these from the repository or prompt the user to pull them from a local source (if there is a security concern on pulling from the internet).     As it is, deployment of agents takes more work than is needed.

2) scripts assume that php7.4-cli is installed/avaliable (debian/ubuntu) to run on the host machine.   observium_installscript.sh for agent install should either install (apt install php7.4-cli) on the machine  OR the scripts should be modified to not require this dependancy.   Otherwise you can deploy scripts which would not be functional across the estate.

3) when observium_installscript.sh is run it modifies /opt/observium/config.php to add $config['poller_modules']['unix-agent'] to the file WITHOUT checking that this is already present.   This should be modified to not append this but to replace this in the file so there is only one instance of this flag.

4) there is no mechanism to update unix agent scripts across the estate in operation.   This in conjunction with #1 should have the option to create a cron job (?monthly?) to pull/update scripts from the repository or local repository (if security concerns prevent direct internet access for the client).  Otherwise there is a lot of effort to maintain large estates for monitoring.