Button updates to allow text/icons to be placed on either side of one another

[mcoker]

placeholder for patternfly/patternfly#2574, will likely result in adding an additional element to the button to be able to better control the CSS

[mcoker]

The work for this is is done in core - patternfly/patternfly#2828

• Removes .pf-c-button__text
• Adds .pf-m-start and .pf-m-end variations to .pf-c-button__icon for when the icon is either positioned at the start or end of the button.