Allow overriding environements on command line

Add a new command line option -e to override environement variables when
running a service.

Signed-off-by: Chmouel Boudjnah <chmouel@chmouel.com>
Use /bin/echo for intermediate container

In cases where the service is using a minimal container,
/bin/echo can be created but echo cannot.

See #517

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge pull request #534 from bfirsh/use-bin-echo-for-intermediate-container

Use /bin/echo for intermediate container
Merge pull request #529 from chmouel/override-environement

Allow overriding environements on command line
Ship 1.0.0

Signed-off-by: Ben Firshman <ben@firshman.co.uk>