Address gpg signing failurs with jenkins build

[Channyboy]

After infrastructure upgrades we're encountering jenkin build failures. Due to :

[INFO] [INFO] --- maven-gpg-plugin:1.6:sign (sign-artifacts) @ microprofile-metrics-parent ---
[INFO] gpg: signing failed: Not a tty
[INFO] gpg: signing failed: Not a tty
[INFO] [INFO] -------------------------

See for example: https://ci.eclipse.org/microprofile/view/Release/job/MicroProfile%20Releases/283/console

The main microprofile umbrella project is able to pass their jenkins build by specying the pinentry mode to loopback See: https://github.com/eclipse/microprofile/blob/8228cb5e7430f44e6a58b569775a3316e4a5844f/pom.xml#L316-L321

Signed-off-by: David Chan chdavid@ca.ibm.com

[Channyboy]

fixes #486