UnboundLocalError when getting config from environment vars only

[Andreas Scheuring]

Getting the following error under the following conditions
* Option set via env variable AND
* option *not* defined in the config file AND
* the validation of its value fails (e.g. as the defined regex does not match).

It works as expected when a the option is defined in the config file. More details in the reproduce section

```
Traceback (most recent call last):
  File "test.py", line 19, in <module>
    print(cfg.CONF.group.foo)
  File "/home/nch7fe/gitt/osn2-na-api/.venv/py27/local/lib/python2.7/site-packages/oslo_config/cfg.py", line 3130, in __getattr__
    return self._conf._get(name, self._group)
  File "/home/nch7fe/gitt/osn2-na-api/.venv/py27/local/lib/python2.7/site-packages/oslo_config/cfg.py", line 2627, in _get
    value, loc = self._do_get(name, group, namespace)
  File "/home/nch7fe/gitt/osn2-na-api/.venv/py27/local/lib/python2.7/site-packages/oslo_config/cfg.py", line 2697, in _do_get
    opt.name, alt_loc, str(ve))
UnboundLocalError: local variable 'alt_loc' referenced before assignment
```

Reproduce
=========

Save the following script "test.py"

```
from __future__ import print_function

from oslo_config import cfg
from os import environ

g = cfg.OptGroup("group")
foo = cfg.StrOpt("foo", regex="^[a-z].*$")

cfg.CONF.register_opt(foo, group=g)

environ["OS_GROUP__FOO"] = "abc"
cfg.CONF()
print("### Correct value set via env variable")
print(cfg.CONF.group.foo)

environ["OS_GROUP__FOO"] = "ABC"
cfg.CONF()
print("### Invalid value set via env variable")
print(cfg.CONF.group.foo)

del environ["OS_GROUP__FOO"]
cfg.CONF()
print("### Invalid value from conf file")
print(cfg.CONF.group.foo)

```

store the following config file "config.cfg"

```
[group]
foo=BAR

```

Now run the script

`python test.py --config-file config.cfg`
-> as expected, getting a `ConfigFileValueError`

now clear the config.cfg file and run the command again

echo "" > config.cfg

`python test.py --config-file config.cfg`
-> Result: `UnboundLocalError: local variable 'alt_loc' referenced before assignment`
-> Expectation: `oslo_config.cfg.ConfigFileValueError: Value for option foo from LocationInfo(location=<Locations.user: (4, True)>, detail='/home/nch7fe/gitt/osn2-na-api/test.cfg') is not valid: Value 'ABC' doesn't match regex '^[a-z].*$'`

Version
-------
python 2.7.15rc1
oslo.config==6.11.0

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/724880

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/724880
Committed: https://git.openstack.org/cgit/openstack/oslo.config/commit/?id=2c11e9dd8221325a1071d42c069dc6c806d3ce60
Submitter: Zuul
Branch: master

commit 2c11e9dd8221325a1071d42c069dc6c806d3ce60
Author: Stephen Finucane <email address hidden>
Date: Fri May 1 16:45:04 2020 +0100

    Resolve UnboundLocalError

    As discussed in bug 1841038, configuring a StrOpt with a regex parameter
    and providing an invalid environment variable will result in an ugly
    stacktrace due to an undefined 'alt_loc' variable. Resolve this by
    creating said variable.

    Change-Id: Id0fb6efdc435a2bd88d5dfea2b4720df453fbfec
    Signed-off-by: Stephen Finucane <email address hidden>
    Closes-Bug: #1841038