Fix: support for dpkg ubuntu

[cgetzen]

Issue #, if available: #798

Description of changes:

Fixes nodeadm init on Ubuntu when amazon-ssm-agent is installed via deb/dpkg instead of snap.

• Detect whether the SSM agent is actually installed from snap before choosing the daemon name, proxy config path, and package manager commands.
• Fall back to the standard amazon-ssm-agent service and non-snap package handling for Ubuntu systems using deb-based installs.
• Add unit coverage for Ubuntu daemon-name resolution so both snap-installed and deb-installed SSM paths are exercised.

Testing (if applicable):

• TestResolveDaemonName has been added
• Manual testing has been performed

Documentation added/planned (if applicable):

None planned, since this is a bug fix.

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