Add a "--profile" parameter to cleanbuild

[chrisglass]

This allows the user to select another LXD profile when running
cleanbuild, in case some profile-specific options are required. This
allows users not to have to change their default profile.

This partially fixes LP:1686766

The change follows the existing pattern (passing an explicit option instead of an "extra options" free-form parameter), as with the existing "--remote" option.

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• Have you signed the CLA?
• If this is a bugfix. Have you checked that there is a bug report open for the issue you are trying to fix on bug reports?
• If this is a new feature. Have you discussed the design on the forum?
• Have you successfully run ./runtests.sh static?
• Have you successfully run ./runtests.sh unit?

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[chrisglass]

I think that failed check is not related to my code.

[sergiusens]

Moving forward, we have decided to move away from containers as the mechanism to execute builds https://forum.snapcraft.io/t/draft-consistent-build-environments-build-vms/5374/3

Sorry for the late reply and the wasted effort.