Allow to configure chroot command

[gjkloosterman]

Allow the chroot command to be configured to something other than "chroot".

What do these changes do?

With this change the chroot command used by exec_helpers can be configured at runtime.

The intended use case is to make it possible to replace the default chroot command with an alternative that makes /proc available in the chrooted environment. Go binaries fail to execute properly otherwise.

Are there changes in behavior for the user?

Related issue number

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes

[coveralls]

Pull Request Test Coverage Report for Build 9316905727

Details

• 13 of 55 (23.64%) changed or added relevant lines in 4 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.7%) to 55.058%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
exec_helpers/_helpers.py	1	2	50.0%
exec_helpers/_ssh_base.py	1	3	33.33%
exec_helpers/api.py	11	25	44.0%
exec_helpers/async_api/api.py	0	25	0.0%

Files with Coverage Reduction	New Missed Lines	%
exec_helpers/api.py	1	55.07%
exec_helpers/async_api/api.py	1	0.0%

Totals
Change from base Build 9314535336:	-0.7%
Covered Lines:	1075
Relevant Lines:	1961

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💛 - Coveralls