Release 1.5.1

Significant changes in this release include:

• pgo label command - an initial implementation that lets you add user defined labels to clusters
• pgo user command - an initial implementation that lets you add users to clusters and manage passwords
• support for selectors in various commands allowing you to perform changes across a set of clusters
• pgo create cluster adds a --series command flag that lets you create a series of clusters off a base name
• initial support for password aging