Update Filter to latest PatternFly designs #553

Currently the filter component:

1. Has the placeholder text within the dropdown, remove
2. Dropdown shows italicized current value. Only placeholder should be italicized
3. Dropdown filter can be applied multiple times, restrict to single select/filter application
4. Dropdown doesn't remember previous selection when switching between filter fields.
5. Dropdown should revert to placeholder when it's filter is removed.

@mcarrano

Related PR: patternfly/patternfly-ng#449