Description of problem:
When two hosts in "filter_host_uuids" with same quotes('' or "") in /etc/virt-who.d/XXX, if one host is not exist, virt-who can't filter the other legal one.
if one host is set to null, virt-who will show error info and stop automatically.

Version-Release number of selected component (if applicable):
virt-who-0.17-6.el7.noarch
subscription-manager-1.17.9-1.el7.x86_64
python-rhsm-1.17.5-1.el7.x86_64

How reproducible:
Always

Steps to Reproduce:
1. Register system to satellite6.2
2. Set "filter_host_uuids" with same quotes '' or "",564D80F1-61F0-8510-87BF-57F9EC6122D7 is legal, "test" is illegal.
# cat /etc/virt-who.d/virt
[hyperv]
type=hyperv
server=10.73.5.212
username=administrator
password=Welcome1
owner=7970632
env=7970632
filter_host_uuids='564D80F1-61F0-8510-87BF-57F9EC6122D7','test'
or 
# cat /etc/virt-who.d/virt
[hyperv]
type=hyperv
server=10.73.5.212
username=administrator
password=Welcome1
owner=7970632
env=7970632
filter_host_uuids="564D80F1-61F0-8510-87BF-57F9EC6122D7","test"
3. Restart virt-who and check virt-who's log
4. Set "filter_host_uuids" with same quotes '' or "",564D80F1-61F0-8510-87BF-57F9EC6122D7 is legal, the other host is null.
[hyperv]
type=hyperv
server=10.73.5.212
username=administrator
password=Welcome1
owner=7970632
env=7970632
filter_host_uuids="564D80F1-61F0-8510-87BF-57F9EC6122D7",""
5. Restart virt-who and check virt-who's log

Actual results:
After step 3: Virt-who can't filter the legal host "564D80F1-61F0-8510-87BF-57F9EC6122D7", it will show null host.
"2016-07-25 04:38:56,550 [virtwho.main INFO] MainProcess(14047):MainThread @subscriptionmanager.py:hypervisorCheckIn:194 - Sending update in hosts-to-guests mapping for config "hyperv": 0 hypervisors and 0 guests found
2016-07-25 04:38:56,550 [virtwho.main DEBUG] MainProcess(14047):MainThread @subscriptionmanager.py:hypervisorCheckIn:195 - Host-to-guest mapping: {}
" 

After step5, it will show error info and virt-who will stop automatically
2016-07-25 04:43:10,874 [virtwho.main ERROR] MainProcess(14113):MainThread @__main__.py:main:16 - Fatal error:
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/virtwho/__main__.py", line 8, in main
    res = virtwho.main.main()
  File "/usr/lib/python2.7/site-packages/virtwho/main.py", line 178, in main
    return _main(executor)
  File "/usr/lib/python2.7/site-packages/virtwho/main.py", line 186, in _main
    result = executor.run()
  File "/usr/lib/python2.7/site-packages/virtwho/executor.py", line 249, in run
    if self.last_reports_hash.get(report.config.name, None) == report.hash:
  File "/usr/lib/python2.7/site-packages/virtwho/virt/virt.py", line 245, in hash
    return hashlib.sha256(json.dumps(self.serializedAssociation, sort_keys=True)).hexdigest()
  File "/usr/lib/python2.7/site-packages/virtwho/virt/virt.py", line 240, in serializedAssociation
    'hypervisors': sorted([h.toDict() for h in self.association['hypervisors']], key=itemgetter('hypervisorId'))
  File "/usr/lib/python2.7/site-packages/virtwho/virt/virt.py", line 231, in association
    if self._config.filter_hosts is not None and host.hypervisorId not in self._config.filter_hosts:
  File "/usr/lib/python2.7/site-packages/virtwho/config.py", line 123, in __getattr__
    return parse_list(value)
  File "/usr/lib/python2.7/site-packages/virtwho/config.py", line 75, in parse_list
    raise ValueError("Unterminated %s sign" % {"'": "single quote", '"': "double quote"}.get(read_to, read_to))
ValueError: Unterminated single quote sign

Expected results:
Virt-who should filter host correctly no matter with the same quotes or different quotes. it also shouldn't show error info but show correct host when filter a null host.

Additional info:
When two hosts with different quotes, virt-who can work normally.