Bug 1223252

Description of problem:
1.when use "osc lable" command to update resource label with empty value, it can success. But when use this label to filter the resource, it tip the label invalid. So we shouldn't update resource label with empty value.
2.When use invalid characters as the label value, the tip should be friendly.

Version-Release number of selected component (if applicable):
openshift v0.5.1-230-g8bbf71f
kubernetes v0.17.0-441-g6b6b47a
devenv-fedora_1572 (ami-0c213864)

How reproducible:
Always

Steps to Reproduce:
1.Get the service under a namespace
$ osc get service -n test
NAME       LABELS                                          SELECTOR        IP(S)            PORT(S)
database   template=application-template-stibuild   name=database   172.30.247.87    5434/TCP

2.Add label with empty value for service
$ osc label service database color= -n test

3.Use the empty label to filter the resource
$ osc get service -l color= -n test
the provided selector "color=" is not valid: unable to parse requiremnt: found '', expected: identifier

4.Add label with empty value for service
$ osc label service database testlable=@#$% -n test

Actual results:
2.Update the label successfully
$ osc label service database color= -n test
NAME       LABELS                                          SELECTOR        IP(S)           PORT(S)
database   color=,template=application-template-stibuild   name=database   172.30.247.87   5434/TCP

3.$ osc get service -l color= -n test
the provided selector "color=" is not valid: unable to parse requiremnt: found '', expected: identifier

4.$ osc label service database testlable=@#$% -n test
Error from server: service "database" is invalid: [metadata.labels: invalid value '@#$%': must have at most 63 characters, matching regex (([A-Za-z0-9][-A-Za-z0-9_.]*)?[A-Za-z0-9])?: e.g. "MyValue" or "", metadata.labels: invalid value '@#$%': must have at most 63 characters, matching regex (([A-Za-z0-9][-A-Za-z0-9_.]*)?[A-Za-z0-9])?: e.g. "MyValue" or ""]

Expected results:
2.Can't update label with empty value
4.When use invalid value, the tip should be friendly.

Additional info: