Fix os_type comparison in azure_rm_manageddisk with existing disk

[nbr23]

SUMMARY

Fix os_type comparison in azure_rm_manageddisk always returning true.

The current comparison was happening between a string (eg linux) and a OperatingSystemTypes variable (eg OperatingSystemTypes.linux), always returning True.
This impacts the value of the changed variable, signaling a change when none was done.

ISSUE TYPE

• Bugfix Pull Request

COMPONENT NAME

• azure_rm_manageddisk

[nbr23]

Bundling d4004ed in as well, which leverages self.compute_models.OperatingSystemTypes(STR) to get the OperatingSystemTypes object instead of the dict base lookup.
Not a game changer but it homogenizes things, and future proofs (if another OS ends up being supported...)

Cheers!

[xuzhang3]

LGTM