Add more validation for OCIMachineTemplate

[joekr]

What would you like to be added:
Now that #55 has been merged we need to work on adding more validation checks for things like the following:

• CompartmentId
• Shape is valid (might need to call to OCI for validation or the OCI SDK might have pre-flight validation we could use. This one might not be straight foreword so some thought should be used on this validation via a webhook or not.)
• NetworkDetails (valid CIDRs)
• IsPvEncryptionInTransitEnabled (some bare metal shapes require this to be false https://docs.oracle.com/en-us/iaas/releasenotes/changes/60d602f5-abb3-4639-aa19-292a5744a808/)

Why is this needed:
Now that #55 is merged we should expand upon our validation. This issue isn't to 100% cover all possibilities, but rather cover what we can now that make sense.

[joekr]

After speaking with the team we've decided to limit how strictly we validate for now. Things like IsPvEncryptionInTransitEnabled are documented and handled on the compute side. Having the client side handle validation will start to become brittle.