Allowing PERFPOOL to be defined by zfs-test users

[kxgreene]

Motivation and Context

When using the performance tests in zfs-test, the pool chosen is hardcoded to perfpool.

It would be beneficial for users to be able to choose the name of their pool. One example is if
the user has ongoing monitoring on a pool name (say domain0), they would be able to
monitor their pool during performance testing by passing in domain0 in an environment
variable.

Description

Allows the PERFPOOL variable to be collected from an environment variable.

How Has This Been Tested?

I have run zfs-tests with a pool name assigned and saw the zpool create commands
create the correctly named zpool.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.